I can't answer about the color. In my area, the busses are all identically painted. However, the size, to my understanding is generally economic. Schools usually don't have as many students using specialized transportation. Most times academically disabled students can ride a regular school bus. The shorter busses are only for students who cannot ride a regular bus, for whatever reason. So a larger bus, which costs a lot more money both to purchase and to drive, is not an economical solution.
I believe it is a practical decision. However, it does tend to single students out, and I have known of students teased because of it.