New plasma update sets the accent colour from your background. To change it go to System Settings > Colours and Themes > Colours (not sure if Mint has it in the same place) then set the accent colour you like from the top.
Oh I see, that is odd. I thought your background was yellow beforehand. Is it perhaps plasma's Night Light? That will tint everything yellow when it's enabled.
![[Solved]What happened to my background/desktop?](https://lemmy.world/pictrs/image/d18231de-d9d4-479e-9a62-165c02174d9a.png){kind=link}