Why are galactic nuclei yellow?
It's an observational fact that many disk galaxies have a color gradient with redder (or as you say 'yellow') stars in the central regions and bluer stars in the disk.
There are two things that can cause this central redness.

Age, older stars being redder than younger stars
Metallicity, with red stars being more metal rich than blue stars

Whether it's age or metallicity or both causing these gradients is topic of current research. It's a popular theory that galaxies grow inside-out, which causes a red center.