Depends on the type of ball you use... and I would not use crayon to fill engravings. Color effect will be great, but it will come off as easy as you paste it into the engravings. There is no good adherence to the ball surface as a true color coating will have.
Personally, I would not recommend changing engravings on balls which have their logos etc. "baked" into the coverstock, like Brunswick or MoRich do. You can get it out with a mini drill, but it will look awful.
Other balls with real engravings like Ebonite, Hammer or Storm are easier to handle. You can either scratch the old paint out, using a tack, steel needle or something similar, or do it professionally with a mini drill and matching milling heads (you get good, cheap ones in DIY markets). Just be careful to stay in the engravings.
For filling, I'd recommend glossy water-based acryllic paint. From my experiences with some balls (a TPC Player which has now Storm-style yellw-ornage-red engravings all over, a TPC Shooter with new neon green engravings and a Blade Particle which received all new white/light blue engravings - sorry, I do not have webspace for posting pictures!), this is the easiest to handle material and has the best long-term persistence and dirt resistance. You will not need any chemicals to clean up the brush, and it will not stink like a nitro based coating which will tend to be sticky - as well as oil paint, which can do a good job, too, but it stays very soft and will get ugly soon.
Two coatings ensure brightness, when you use light colours like yellow, make sure to have a layer of white (best matte for improved binding with the coverstock) below.
Use a good hobby painter brush, I recommend investing some bucks into a brush with mink hair - it has the best tip for exact working.
Excessive paint can dry up and be rubbed off with a soft cotton cloth without chemicals etc..
Good luck. It is worth a try and can give an old ball truely good looks.
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DizzyFugu --- Reporting from Germany
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