|Without getting to into too lengthy an explanation:|
- different colored slots means the motherboard supports dual channel mode, provided you use the correct slots.
- the proper RAM for this system is DDR2-800 (aka PC2-6400) regardless of the type HP originally installed. And it MUST be RAM that runs at 1.8v...not 1.9v, 2.0, 2.1v, etc.
- ALL RAM, old or new, should be tested with memtest86 before being put into service. The sticks should be tested individually, then together.
- working RAM generally doesn't "go bad" for no reason. The error message on the blue screen is there to tell you what is wrong so that you can fix it. What message did you get that indicated the RAM was bad?