I'm wondering why are part of my gums bright yellow?

See a dentist. I can not think of a reason why your gums are yellow. They have lots of blood vessels and should be a shade of red. If you have very thick bone pushing and thinning the gums out they might appear to be less red.
Plaque/deposits. Could deposits on your teeth. No matter, see a dentist to check out your current condition. Don't wait.