Scott 1940, 1981 Christmas Issue Missing Yellow

A curious find in a kilo-ware pile.

Would it be possible to produce a stamp like that during printing or it is just a color changeling?

Color changeling most likely. Spent too much time in the sun.

Florida version

You need to take a close look with a good magnifying glass. This stamp was printed in yellow, tan, red, green and brown. Use this a checklist and look for pixels of color for each of those colors. If there is at least one pixel for each of the colors it is not a color missing stamp, but most likely faded from long exposure to a strong light. Yellow is one of the more fugitive colors in that time period.

Or take a similar, unfaded stamp and place in the windowsill. It should take only a few weeks of full sun before it becomes a color changeling.
Don

If you look at the back of the bear, above and below the key I think you see a hint of yellow.