You can use regular washable markers (thin or thick) to create watercolor-like paint.
Scribble some marker onto a non-porous surface, like a piece of plastic wrap/foil/plate (not paper)/paint palette... then you can use the 'marker-paint' like you would normally use watercolor paint, mixable/dilutable.
You can also scribble randomly on a larger non-porous surface and then lay your paper down onto the 'paint' - try with dry paper, then compare with damp paper...you can cover large background areas super quick!
You can do it the opposite way as well - scribble onto the paper, then use a water-filled brush to turn the marker into more of a paint-like look...either as a background or as coloring an image.
Crayola 'Super-Tips' work better than basic/off brand washable markers....but they all work!