If you are seeing some avatars as solid colored blobs, that means they are too "complex" for your viewer to render normally, based on your default viewer settings. This is called "avatar complexity".
The best explanation for this, and how you can manage it your end, is given on this video:
Copy the link, and paste it into your web browser to view it.
Please note that the specific directions on where to find viewer settings will vary based on what viewer you use. Those in the video are for the standard Second Life Viewer. For other viewers, you should ask in the support group for whatever viewer you use.