A material includes such properties as a main colour (‘diffuse’), the colour of reflections (‘specularity’), opacity (alpha). Opacity is the opposite of transparency, so a material with an alpha of 1 is completely opaque (not transparent at all) whereas a material with an alpha of 0 is completely transparent. Materials can also include textures, such as a pattern of bricks or a wood grain texture.
The materials panel is, in my opinion, poorly designed, for it is not at all obvious how to create materials and assign them to objects or faces. You should think in terms of faces rather than objects for it is possible for different faces of the same object to have different materials assigned to them. The most important thing to understand is that you will be making a number of materials that will always be available to every face of every object. When you change a material the changes will affect every face of every object that uses that material.
In fact, if you are working in edit mode so as to be able to assign different materials to different faces of the same object, you first have to assign the materials to 'slots' and then assign slots to faces. Also, in versions up to 2.79 and maybe beyond, the materials don't always show correctly in 'solid' view with Cycles render - that's easy to fix by choosing 'materials' view rather than 'solid' view. This 11 minute video should make these points clearer:
To work with materials, first open the materials panel (see upper image right). Before we go any further, imagine a complex scene with many objects, each object having its own set of materials (though some objects might share some materials). When you select an object, you probably want to see only the materials that are used by THAT particular object - you don't want to see the complete of all the different materials used within the scene. That brings us to the 'slot' panel which shows only the materials used by the currently selected object(s). Actually that's not quite right - it can also have slots containing materials that are NOT currently applied to any faces of the object, but that you have chosen to keep to hand in case you want to use them later (a bit like substitutes in a football team).
So the slot panel only shows a subset of all the materials that are in your project. To see the complete list of all the materials in your project, click the Browse ID Data button (see lower image at right).
To make a new material, first click the Add New Material button (the plus sign next to the X, not the other plus sign). If there are no slots then a slot will be created with the new material. If one or more slots exist then the selected slot will be given the new material (I think this is not intuitive - it should be possible to create a new material without changing the material associated with the selected slot). It is a good idea to rename each material that you create. You can assign materials to faces either in object mode or in edit mode, but it’s probably easier to understand what is going on in edit mode. Select an object in object mode and then switch to edit mode - all the faces of that object should be selected. If you have several objects selected when you try to work with the materials panel then only the active object (the last one you selected, which appears surrounded by a lighter orange outline) will be affected.
In the materials panel is this area that I will call the slots window or slots panel:
My image shows three slots which each contain a material (renamed yellow, red and blue) that can be assigned to selected faces by clicking the Assign button (materials are applied to faces rather than vertices, so you should be in face select mode when you want to select faces to assign to a slot). In the above image the third slot containing a material called ‘blue’ is selected so if I were to click the Assign button this is the material that would be applied to selected faces. So you can see that when in edit mode there are two steps to the process: first assign a material to a slot and then assign a slot to the selected faces. The slots window may contain any number of slots, or none at all. The materials displayed here will be a subset of the materials included in your project. For example, you might have created twenty materials in your project and the slots window of the currently selected object might contain three slots and thus three of the twenty materials. It is possible that different slots of the same object might contain the same material like this:
This would mean that one set of faces has been assigned the material ‘yellow’ that a second set of faces has been assigned the material ‘blue’ and that other faces have also been assigned the materials ‘blue’. This isn't necessarily a mistake, for it means that by changing the material attached to one of the 'blue' slots you can change the color of some blue faces while leaving other blue faces unchanged because they were assigned the other blue slot.
The contents of a slot can be changed to any of the materials existing within the project by clicking the ‘Browse ID data’ button which will cause the full list of available materials to be displayed so that you can choose a material to occupy the selected slot. You can add a new slot by clicking the ‘+’ sign that is above the ‘-’ sign. You can then change this to any available material by clicking the Browse ID data button as just explained), or you can create a new material for the slot you just created. You can also remove a selected slot by clicking the ‘-’ button. If this slot corresponds to any faces in the selected object then they will no longer have a material assigned to them. If you click the ‘X’ button then the selected slot will still exist but will not have any material associated with it.
As previously mentioned, when you change a material the changes will affect every face of every object that uses that material. If more than one object is using a material then when that material is selected a number appears next to it as in the mage at right. If you want to change this material for the selected object without changing it for other objects then click this number – this will in fact make a copy of the selected material with a new and unique name. Different materials cannot share the same name.
As previously mentioned, it is possible to work with materials in both edit mode and object mode. What happens if you work with materials while in object mode? You can do some of the same things you can do in edit mode – you can add and remove slots and you can change what material is associated with each slot (and thus with the group of faces that are assigned to that slot) but you cannot change what faces are assigned to what slot.
A final note: you cannot completely delete a material during a Blender session but if a material is not being used (if it has zero users) then it will be deleted when your Blender session ends. If you want to keep the material available even though it isn't currently being used then you can create a 'fake user' for the material by clicking the 'F' button - this will trick Blender into thinking that the material IS being used so that Blender does not delete it when you close Blender.
A full list of all the materials in your project can be dropped down by hitting the Browse ID Data button . To apply material to a selected object there must be at least one slot in the slots window. An object can have multiple slots each with its own material (multiple shots may contain the same material, of you wish). To apply different materials to different faces of the same object you must be in Edit mode - an Assign button will appear at the bottom of the slots panel when you are in Edit mode. Each slot can have a material associated with it and can be applied to the selected faces by clicking the Assign button. For example your project might contain twenty materials and a certain object might have four slots. Three of these slots might be assigned to different faces of the object, with the fourth slot being kept available just in case it is needed later. Note that in this case the material in the unassigned slot is considered to be 'used' by the object even if no faces use that material - therefore the material will not be forgotten when blender is closed.
To apply a certain material to a certain group of faces in an object:
- Select the object
- Go to Edit mode
- Open the material panel
- If the desired material does not already exist in your object then create it
- Create a slot in the slots window that will correspond to the set of faces and to the desired material.
- Assign the correct material to the slot if it is not already assigned.
- In Face Select mode, select the faces that are to be associated with the selected slot and click the Assign button.
Here is a 7 minute Vimeo video that shows how faces can be coloured in Blender. It uses the Blender render engine but the process would be similar in Cycles render except that you might see incorrect colors in solid view so you should work instead in materials view. You can see faces being painted in Cycles in the first four minutes of this video.