The Working Set column, is that RAM? And if so is that how much RAM is being used by that process?
Yes. The working set of a process is the virtual memory of the process that is in physical RAM.
Note: some of this might be shared with other processes, so the sum of all working sets will be greater than the total RAM allocated to processes.
What do the colors represent? Yellow, Purple, Red, and No Color or Gray.
Options | Configure Colors to see the definitions.
What are Private Bytes?
The amount of virtual memory allocated to the process, it is likely that some of this is in the page file and some in physical RAM.
I would recommend Windows Internals for a detailed description of Windows memory management (all simple explanations are approximations and likely to lead to wrong conclusions). Also Windows Sysinternals Administrator's Reference for details on using Process Explorer and the other Sysinternals tools.