What do your Rules say regarding aesthetic appearance? Do you have those standards in place and did they sign a lease that integrated those rules?
1st) The rules I have are when exterior modifications need to be done, they need to be approved by management. This includes a wide variety of standard color choices from a supplied color chart (you could look through one for exterior homes from home depot for instance). From this they can choose whatever color they want.
2nd) The law in Maine does not allow me to force a color change on a tenant if it will cause undue financial hardship. However, if the home needs to be painted anyway because its peeling, mismatched siding, or repairs to siding make repainting necessary, then the tenant, according to the rules, is supposed to get management approval. If they fail to do so and then go ahead and paint it without management approval in the paint it neon orange, then yes you can notice them for a violation and require them to fix it or potentially be evicted.
All that said, If they did a good job (workman like quality), and the colors are not extreme, you might not make to big a deal of it. However, the next time they need to paint their home as part of keeping up with maintenance standards, they should ask for approval before painting, if they do not they are violating the rules where they are not getting management approval for exterior changes.
You could also offer to repaint their home on your dime if is that horrible.