That is not a sign of pregnancy. It's possible you just never noticed the skin around your vagina in that much detail before; colour variations are quite normal. Also, the tissue will get darker with increased blood flow as you get aroused, so it's possible it's still slightly engorged. Finally, it could be some light bruising, which can happen.
Again, not a sign of pregnancy!
If you're that paranoid and really cannot cope with being pregnant, you might want to check out these two articles:
Ready or Not? –The Readiness Checklist
What's The Risk? - Five Easy Pregnancy Risk Assessment Flow Charts
Am I pregnant?)
Margaret Sanger’s Disneyland - An easy chart of your birth control options
Hope this helps,