And I noticed that the RGB leds have a common anode (positive) and 3 different catodes (negative), so how can I connect two of these leds to 12v and the other to 6-8v if they have the same anode?
Could I connect positive to 12v and then connect negative of Green and Blue to 0v (ground) and Red to 6v? is this correct? I'm thinking I will have 12v-6v=6v on Red led. Is that correct?
That's theoretically correct but, again, it's not appropriate to drive LEDs without current limiting. The best solution is a constant-current LED driver.