From the SA:MP wiki:
A chattext or player color looks like this 0xRRGGBBAA The RR is the red part of the color, the GG the green and the BB the blue. AA is the alphavalue. If you use FF there it will be displayed without transparency and if you put it to 00 it will be invisible.
The colours are in hexadecimal format, so RR is between 00 and FF. A-F represent numbers between 10 and 15. They can be combined to form something like 0F, F0, FF, or 00.
You can use this color picker to make colors:
http://www.developingwebs.net/tools/color.phpAs the wiki says, the AA is an alpha value, so FF will (or should) make text completely opaque, and 00 will make it invisible. Add this to the end of the color you get from the color picker.