The rule is:
colors (green, blue, black, red, sometimes purple)
metals (silver/white, gold/yellow)
- no color on another color
- no metal on anther metal (exception: the Kingdom of Jerusalem: http://en.wikipedia.org/wiki/Kingdom_of_Jerusalem / and the Vatican State)
because you can better recognize on a distance an black eagle on a yellow or white field than on a green/blue/red/black shield.
If your german isn't that bad Jens try this page: http://www.dr-bernhard-peter.de/Heraldik/farbregel.htm
But you're right ... there some other exceptions (but heraldic wrong) and some other colors like grey, brown, flesh and you have additional furs like ermine/vair/potent.
Reglen for farvelægning er, at man skal veksle farverne med metal, det vil sige at opdelinger ikke må være guld/sølv eller blå/rød, men gerne guld/rød osv.
Ligeledes skal en farve sættes på metal eller omvendt. En guldløve sættes ikke sølvbund, men på blåt eller en anden farve.
Reglen er til for at skabe kontrast i skjoldet, så symbolerne kunne ses på lang afstand.