Hi Jennifer, thanks for getting in touch!
We have 
route_color and 
route_text_color information available in our GTFS feed, which you could more easily access through 
the /routes endpoint in our V3 API. This would give you a list of all our routes, with hex color information listed for each. These colors vary by mode, and these are the colors you'll see reflected on Google Maps, Transit App, our website, and other third-party apps. You can also view our brand colors (with hex codes) in 
our brand guidelines, here. 
However, if you're talking about 
our full system map, which assigns different colors to different bus routes for readability, unfortunately we don't publish data about those colors. 
Is this info helpful for your needs? Let me know if there's anything else we can do to help. 
All the best,
Developer@MBTA