If the argument to "-c" is optional, the parser doesn't know if "blue" is a an optional argument to "-c" or separate argument to "program". I think the takeawa

If the argument to "-c" is optional, the parser doesn't know if "blue" is a an optional argument to "-c" or separate argument to "program". I think the takeaway is to avoid options with optional arguments. If the argument to -c was required, "-c blue" would be fine.