I just want to make sure I am understanding this correctly. In the sentance " Ich mag grunen Tassen"
it is "grunen" instead of "grun" because there are multiple cups. So is it anytime you are discussing the color of more than one thing you conjegate it like that?
I'd like to enhance kyky's examples by giving a few more and depending on the gender of the word:
"Ich mag die grünen Tassen" (die), "Ich mag die grünen Autos" (das), "Ich mag die grünen Stühle" (der)
"Ich mag grüne Tassen" (die), "Ich mag grüne Autos" (das), "Ich mag grüne Stühle" (der)
As you can see, there's no dependency on the article what makes it quite easy.
But: Actually you only asked for colors in plural, but I find it important to mention, that you have to pay attention to the article in singular!
"Ich mag die grüne Tasse" (die), "Ich mag das grüne Auto" (das), "Ich mag den grünen Stuhl" (der)