Google Maps KML: 8-Digit Hex Code

You can use 6 digit hex code in reverse for the last six characters. The first two characters represent transparency which can be set from 00 which is invisible to ff which is opaque.

This is what I use: http://www.zonums.com/gmaps/kml_color/

(this page lets you enter the code)

The hardest part is remembering the order of the pieces (from the spec):

The order of expression is aabbggrr, where aa=alpha (00 to ff); bb=blue (00 to ff); gg=green (00 to ff); rr=red (00 to ff).

The first two digits ("aa"/alpha in aabbggrr) represent the opacity.