While taxi color in other countries signify nothing in particular, Bangkok's taxis are actually color coded.
The single-color are company taxis, personal taxis in cooperation or alliance and rental company taxis. These color include bright green, bright sky blue, red, orange, yellow, blue, pink, purple, violet and tan.
The bi-colored taxis are in 3 kinds including yellow-green, red-blue and yellow-orange. The yellow-green are the personal private taxi. The red-blue are the rental taxi. The yellow-orange are the company taxi.
Taxis are abundant in Bangkok so you can pick any color you like, but the yellow-green taxis are generally reckoned to be better, being owned and driven by the owners themselves.