I hesitate to call this a bug, more likely it is an oversight: normally when you type color() in the editor, you can then tap on the () part for the color selector. This is not available, when you type color() as a function argument. It is available, however, when using color() inside a table constructor.
Yes, that is a bit annoying, I usually make a line break, and then it works. 
Same thing with sprite() and readImage(). There may be others. I also create a second line the back it into the line above it and then complete the syntax.