This one frustrated me for quite a while. I experimented with pallets, colour options and anything else in the menus that had to do with colour. Nothing helped. Yellow was always a light custard colour. Google rescued me and I found the link below.

Apparently photoshop can forget setting or a corrupted config file screws things up. So I clicked the Photoshop icon then quickly pressed CTL-SHFT-ALT. A Dialog box appears confirming that you want to reset the config file or something to that effect. And that fixes things. Be warned though, any custom settings or brushes you have will be lost.