Why not add the b instead of editing the number? Obfuscation perhaps?

In your loop for actual color settings you could include the attribute setting also:

FOR i = 0 TO 15
IF colors(i) THEN
OUT &H3C8, i
OUT &H3C9, ASC(r) \ 4
OUT &H3C9, ASC(g) \ 4
OUT &H3C9, ASC(b) \ 4
END IF
NEXT

You don't need to use OUT &H3C8, i when you are not changing a color.

In fact, you can skip it if every higher color setting is changed. Every three &H3C9 statements go to the next attribute. When you do that, you only need to specify the lowest attribute at the beginning with &H3C8, ?.