My color management seems to have gone crazy. I have a Dell 2005FPW monitor, and I usually have the RGB setting in color settings/working spaces/RGB set to: 'monitor RGB - Dell 2005FPW Color Profile. Without me changing anything, white now appears yellow on my screen, even though when I look in color picker and select white, the numbers are correct - 255,255,255 and #ffffff. I have tried changing to other settings but white still looks yellow, although white appears white when I print.
>> I usually have the RGB setting in color settings/working spaces/RGB set to: 'monitor RGB
That is never recommended here — try setting a device independent Working Space like "sRGB 2.1..." as your Working RGB (until you better know what you are doing).
>> white now appears yellow on my screen, even though when I look in color picker and select white, the numbers are correct - 255,255,255
That is a sure sign of a bad monitor profile in Photoshop, you need to reprofile your monitor and get it engaged in System Preferences> Displays> Color> Display Profile (or Windows equivelent) — Photoshop/Adoe apps will pick it up from there for color-management purposes.
>> I have tried changing to other settings but white still looks yellow
I don't know what that means, 'whites appear yellow' is a FAQ...
Try these settings:
North America
Europe, Middle East and Africa
Asia Pacific