If I'm designing af tool that must "screenshot" well for printed documentation, can I easily choose colors that look different even when printed in greyscale?
EDIT: I was hoping for some easy-to-use palette or tool, but the inputs given already is very insightfull for sure
Yes. Your best choice would be to choose colors that have a high level of relative contrast. Frankly, it might even be easiest for you to design your UI in greyscale in the first place. Basically, you're going to want to choose colors that are either lighter or darker than the colors around them by a decent amount.
You could calculate the luminance from the RGB values:
Y = 0.2126 R + 0.7152 G + 0.0722 B
And make sure your Y values for your selected colors are as distributed as evenly as possible.