Because they had low oxygen levels after the vehemence with which they massacred each other?
Your dream is weird.
Why would they fight over mask fabric? They all wear different colors. It's the one thing everyone knows about the Teenage Mutant Ninja Turtles, besides the teenage, ninja and mutant part. And the fact that they are turtles. That's actually a lot. It's pretty much the fifth thing people know about them. It goes:
5: Different Colored Mask Fabrics
6: Turtle Power
Did Donatello have some particular significance in your dream? Is he your favorite ninja turtle? His mask is purple. See what I'm doing?
I guess your question brings up a lot more questions than it does answers. Are you okay? Do you often have dreams where foursomes murder each other? Or where cherished, childhood things get into arguments that could be solved by a trip to the Sewing Nook? Or where Prince is about?
Short Answer: Dreams are hard to answer, so instead I'll do what I want. Michelangelo was my favorite turtle until Michael Bay took a huge shit all over him and made him into an absolute moron. Now none of them are my favorite. That's the Bay effect. I don't have a favorite Transformer anymore, or a favorite Meghan Fox, either. And I'm pretty sure he's responsible for turning a promising young Shia Labeouf into...whatever he is now.