is it legal to paint on it?

Plates cannot be modified in any way. Even trimming them to fit some types of frames/covers is illegal.

If you want to paint it because it is faded the Land Transport Office will replace defective plates for free until September. There was a large batch of defective plates issued a couple of years ago

You just go in and fill out a form, taking your Blue Book and they will notify you when your new plates are in (takes about a month)

The licence plate of the missus was going a bit vague tho. We just used a black marker pen and its still on for some time. Didnt know they were giving away free licence plates but its a bit more hassle then just using a black market pen for some touch ups

I would go for getting the replacement plates since they are free and the delivery time is more reasonable. The 'less hassle' method of touching up the plates may get the attention of a picky traffic cop on a routine traffic stop. They don't need much of an excuse to ask for money.