Riddle Me This

I have never been a plum eater. Once Luis started on solid foods, I was eager to expand his eating horizon as much as possible. Plums was a food that several people suggested. So I bought half a dozen at the grocery store, steamed them, pureed them, and froze little cubes of dark red plum puree. Luis loved them. A couple of weeks later, I bought more plums. This time, they were a yellow/orange color, almost like peaches. I was mystified. They looked the same on the outside...

I asked two plum aficionados that I know, and they didn't have an answer for the differences in the plum batches. Today, I made a batch of plums for Luis. They were of the dark red variety. Except the last one (the middle one in this picture). Anybody out there know what's going on here? I'm quite curious (but not curious enough to google it myself)...