Sorry, primer is a must. It's cold here, and if you don't prime first, that paint will likely peel up in the spring. If you choose not to prime, then I'd just choose not to paint at all.
When painting wood that's exposed to outdoor temperatures, I like to use oil based kilz primer - it will seal the wood well, hold up to cold temperatures, and will hide any knots in the wood (that will bleed through your paint in a nasty brown spot after it dries). You can then use latex (easier to clean up) exterior paint for the color choice. Yes, I said exterior. It gets cold here and interior paint is not formulated to get down to our low temperatures. Interior paint will hold up in the short run, but will need to be redone sooner. I definitely agree to use the "oops" bin - plenty of good choices there if you want to save some dough. Go with a semi-gloss or gloss for washability. I'm not sure if there's a certain color that chickens seem to like more. But lighter colors will brighten up the inside of your coop, extending any sunshine that you get from windows. I'm with the other posters -- go with a color that makes GABBY happy. If a bright color is on the wish list, paint one wall with that one. I have ice blue walls and one wall, by their roost, is brown. I like it alot.