Krylon spray stains work great to color sections of clear ropelight, as folks above posted. They allow a lot of light to pass, unlike paints like Fusion, etc.
Here's a little tip I have been using for quite a few years when staining clear rope - the colored stain will dry much faster, and not remain tacky for days, if you first spray the rope with clear krylon/similar paint.
It bonds to the plastic surface, and drys quickly. The stain is then applied over it, and drying time is about 25% of what it otherwise would be. There is no additional impact on light transmission.
Say goodbye to sticky ropelight/smudged fingers!
Greg