Were Yield signs ever yellow?

Yes - in fact, yellow was the standard color for YIELD signs for nearly 20 years.

The YIELD sign was added to the Manual on Uniform Traffic Control Devices in 1954 to assign right of way at intersections where a stop was not normally required. The sign was established as a point-down equilateral triangle with black legend and border on a yellow background. Some agencies also used a "keystone"-shaped YIELD sign, and others added the text "RIGHT OF WAY" in the space below the YIELD legend.

In 1971, the YIELD sign was changed to use the red background you see today, along with the white region in the center of the sign.