I read that the first recorded use of plum as a colour name in English was in 1805. The source was Colour Dictionary by A. Maerz and M. Ren Paul - book published in 1930. Sadly, I could not find a copy of this book in our library system.
I was introduced to Mirabelle plums while in Paris. I've not found yellow plums where I live, ours are purplish or reddish in colour.