lualoader: screen argument fixes

screen was also guilty of not-so-great argument names, but it was also
guilty of handling color sequences on its own. Change those bits to using
the color module instead.

As a side note, between color and screen, I'm not 100% sure that returning
the color_value is the right thing to do if we won't generate the escape
sequences. This should be re-evaluated at a later time, and they should
likely return nil instead.