On Multiload, add these pokes to location 64652 when using the GREEN/BLACK option:

0 = All masked black
1 = All black / blue
2 = All black / red
3 = All blue / red
4 = All black / green
5 = Black / cyan then blue / green
6 = Green / red then yellow / black
7 = Red / cyan then blue / yellow

Other numbers give the same results, or do not allow loading at all, so it is best to just stick with these.