If you are using a Garmin marine unit then you are limited to the colour range defined by Garmin, which is broadly the same as the colour range you will find on normal nautical charts. This is of course very sensible, particularly if you are moving between charts.You should have 5 blues if I remember correctly.
If you have a non-marine unit then you can define colours and have a huge selection available. You should get the cgpsmapper manual to understand custom Garmin objects, then you will need to rewrite the scripts in GME. Its not exactly trivial.