For whatever reason, the designers of the AlphaSmart Neo selected an ugly military green for the case. I suspect that AlphaSmart later regretted that decision, since none of the marketing photos for the device portray the real color.
After reading a few articles about vinyl dye being used on computer case mods, I decided to try the technique on my AlphaSmart. I used SEM Color Coat "Flame Red" for the top half of the shell and "Satin Black" for the bottom half. This stuff is usually used to re-color (or restore) automobile interiors. The SEM product is not technically a dye, but it behaves very much like one. It's inexpensive, too–about $8 a can. I ordered from one of the Amazon car parts stores, but you can probably find it at other shops that carry automotive supplies.
It really turned out well. The Color Coat completely covers the plastic case, seeping into every surface feature. (Spray paint often fills these areas, hiding the details.) The Neo truly looks like it was manufactured in this color.
If I were to do this again, I'd probably choose a slightly darker red–one that doesn't scream, "Hey, look at me!" so much. SEM's "Portola Red" looks about perfect for my AlphaSmart Dana.
Note on the keyboard: Rather than try to come up with a tactic for dying the keys and then relabeling them, I used a replacement keypad for the AlphaSmart Dana. The keys are a very dark blue, almost black, so they seem to match the flame red quite well. The Dana keypad is 100% compatible with the Neo, although there are now a few Dana-labeled keys for which I've had to memorize the Neo-specific functions.