Hi, I'd need to know more about rgbcols. My instinct is that you're getting this because rgbcols is a list, and so paste(rgbcols[t]) is a list; colour usually needs to be specified as a string (see the 'Color Specification' section of ?lines).
Please share a reproducible example which isn't necessarily based on your skydat data (so maybe make an example with a built-in dataset like iris instead of your Tracer data) and give detail on what rgbcols is. Hope that helps,