What do the colors mean in htop?

Press h inside htop for quick help.

CPU

Blue      : Low-priority threads
Green     : Normal priority threads
Red       : Kernel threads
Turquoise : Virtualization threads

Memory

Green         : Used memory
Blue          : Buffers
Yellow/Orange : Cache

Also, in the list of processes, there are some that appear in Green. The black ones are main processes and the Green ones are threads. In my example below, I'm running a repair on a Cassandra node and as we can see most entries are threads.