Here's a picture of the paint scheme
Everything is straight lines except for the part on the nose. If you cut the masking tape right, you can easily paint that. Or, you can eyeball it and fix it with rubbing alcohol if you mess up.
For the numbers, you can either get decals or paint them yourself.
If you want to paint a F40PH, (the screamers, 1000-1017. The F40PHs without a seperate HEP generator. The HEP comes directly from the prime mover) it's just a purple stripe all around.
The F40PH-2Cs are pretty much the same color scheme as the GP40MCs.
I am leaving the F40PHM-2Cs (1025-1036) out of this because they are a little different in shape. They are F40 clones built by Morrison-Knudsen. But otherwise, they have the same color scheme as the F40PH-2Cs (1050-1075).
If you want to do a historical model (I am suggesting this because you have a Boeing LRV on the layout which is no longer in service), you might want to consider getting a F10 and repainting it in MBTA colors (they had a couple color schemes). The F10s were retired/sold to CCRR in the 80's.
I would not paint my own numbers. they are to small for HO.
Also, F10s are available from IHP if you want one.