Never heard of that color, but my recent experience tells me that if you find someone that has a bottle you have a good chance that it is still good, as long as it was the regular floquil faint and not Polly-S. I have a box full of old bottles of floquil, most of them are about 15 years old, and the vast majority of them were still in perfect shape...some had never been opened, which may have helped some.
I can't say the same for some of the other paints I had. I had to throw out all the Badger Accu-Flex paint I had, as well as most of the Floquil Polly-S and some of the Testor's paints I had. I did have a few bottles of the regular Floquil that had solidified, but not many. I was very surprised, because this stuff has been stored in all kinds of undesirable conditions, including out in an outdoor storage room, and in a hot garage both for considerable lengths of time.
In Floquil's "Painting Miniatures", a booklet published 1961-1963, there is a color comparison listing equating Floquil identical paints except for name and how listed. For example, on page 11 of my edition, there is a heading "Regular Colors" which contains 2 columns, one labeled Flo-Paque and the other labeled "RR Colors". Flo-Paque's "purple" is equal to RR Color's "diesel purple".
So, at least back in 1963 the color, if only the term, "diesel purple" was used. Have no idea when it ceased.