Shall we throw an exception or a warning when a color is missing?

[LucaMarconato]

Thanks for the update. One note: I think that we should throw a warning when the color can't be found automatically, as otherwise the user would silently see a gray plot.

Originally posted by @LucaMarconato in #268

Originally I thought about a warning. @ivirshup reached out suggesting throwing an error. Currently we don't do either and we silently show a gray plot.