For the CPU cores, I think that blue means nice, green normal CPU use and red I/O. But I'm not sure and I haven't found a definite answer.
Then there are the colors for memory. What do green, blue and yellow mean there?
Press h inside htop for quick help.
Blue : Low-priority threads Green : Normal priority threads Red : Kernel threads Turquoise : Virtualization threads
Green : Used memory Blue : Buffers Yellow/Orange : Cache