Here's a screen shot to show an example of the problem.

From time to time, I notice colors missing from the palette that runs along the bottom of the Inkscape window. I'm not sure if these are called swatches, or if swatches are something different? In this case, yellow is missing.
When this happens, I can't use that color. Nothing happens when I click it. Right click doesn't bring up the context menu (Set stroke, Set fill). And mousing over does not produce the tool tip with the color's hex code.
Most of the time it's not a problem. But if a color disappears that I happen to need, then it's a pain. Yes, I know I can use the Fill/Stroke dialog, and in fact that's what I do.
I don't know what causes it for sure. I think it may happen more often if I'm working on a very large, or I should say and image that needs a lot of memory, not large in size. But I haven't nailed this down for sure.
Is this a problem with my system somehow (memory?), or is it an Inkscape issue?
Is there anything I can do about it?
Thanks for your help
