I had one of those lenses like yours and I had several 12x18 enlargements made from its files and they looked awesome. It has some chromatic aberrations but it is a great lens.
What immediately caught my attention was your "suddenly turned yellow" statement. I suspected that you had set the wrong WB in camera but that has been properly addressed already.
Many photographers shoot RAW because of the flexibility of the files but usually expensive software is needed for editing. You can shoot RAW with a tungsten WB and it is a piece of cake to change the WB during editing.
When using JPEG files the camera imbed the WB set by the photographer and then if it needs to be changed it has to be done by Photoshop users with the Color Balance filter available in Image>Adjustments.
I am sure you are relieved that there is nothing wrong with your lens.