How can you make a splash on a yoyo that you can see on the outside while it isn’t clear? I have a throw monkey that is dark red and would not be able to see through on the outside. Also how do you attach a yoyo to the drill? 
attach a yoyo to a drill by the axle. just unscrew it, stick half in, and you’re good to go.
not sure about painting something not clear, never tried it.
you could just try stuff though. like to get a splash, just dip a paint brush in some paint, and splash it on. (just guessing that’s how you’d do it)
You could try dying the yoyo with a thick splash design using some Elmers Glue. I tried it with a proyo. I recomend using a strong thick amount of glue to make it easy to peal off, and get lighter color like baby blue to give it a stronger contrast.