Well to use a color in gimp just open up the color chooser and pick a color
You can pick and choose or define it in HTML style format (e.g 0000FF = Blue)
If the issue your having is with the colors being semi or completely transparent then
what you should look for is the "Opacity" slider which determines the translucency of the color (0-Transparent 255-Solid)
edit: After rereading your question I thought maybe you want to know how to get Primary colors and nice solid shades of Grey.
Doing this is easy if you use the HTML notation described above and understand the principles of mixing colors.
The notion is in the form RR GG BB where each of the RR GG BB can be between
00 and FF (This is hex notation or base 16)
So knowing that FF is full color and 00 is no color we can create our own colors easily:
Red:
FF0000
Green:
00FF00
Blue:
0000FF
Shades between white and black are also extremely easy just reduce or increase all three colors at once by the same amount to get different shades of Grey:
|- FFFFFF <- White
|- E0E0E0 |
|- D0D0D0 |
|- C0C0C0 |
|- B0B0B0 Light Grey
|- A0A0A0 |
|- 909090 |
|- 808080 Mid Grey
..... |
|- 303030 |
|- 202020 Dark Grey
|- 101010 |
|- 000000 <- Black
