This isnt' a rockbox question, just so you know
That's actually not what he asked, but you can use that screen to explore what the letters mean.
Think of them as "numbers" from 00 to FF, where 77 is in the middle. The numbers just go 0-F instead of 0-9 like you're used to, so the first digit is the "tens place" and the second is the "ones place" just like normal.
The first two are "how much red," the second 2 are "how much green" and the third two are "how much blue" which is why they are collectively called RGB (Reg Green Blue). So pure red would be FF0000, pure blue 0000FF, and purple is FF00FF, etc. If you instead do 770000, you get a darker red, and FF7777 is a pinkish color.
Really, that's it.