Congrats, OP! Good luck to you! Colby is an excellent school; I think you’ll like it. Often we imagine ourselves in one kind of setting and find that we can thrive in many.
I don’t think Posse has hardcore income guidelines, but I do think the program is intended for low-income and/or students of color. The original story behind the formation of Posse was a young man who went off to an elite college from a disadvantaged background and dropped out within a year or two. When asked why, he said he never would have dropped out if he had had his posse with him - indicating the need for social support for students from disadvantaged backgrounds to get through a world that is quite alien to them in many ways. Also, these are the first two of three goals of Posse:
To expand the pool from which top colleges and universities can recruit outstanding young leaders from diverse backgrounds.
To help these institutions build more interactive campus environments so that they can become more welcoming institutions for people from all backgrounds.
So although Posse isn’t explicit and doesn’t require any specific income or racial/ethnic background, their goals definitely revolve around increasing diversity in elite colleges (and college graduates in general). Most of the Posse scholars I worked with when I volunteered for the program were students of color and low-income.