what does it means when someone paints his right hand nail with red varnish? (like Murdoc from the Gorillaz)?

Answer 1

Penn Gillette (from Penn and Teller) has a similar thing:

What is the significance of Penn's one red fingernail (left ring finger)?
There are three common answers to this question.
1) It means he once shot a man for asking personal questions
2) When Penn first began performing, his mother told him to get a manicure because people would be looking at his hands. In response to this, Penn had all of his nails painted red as a joke. The one remaining red fingernail is in honor of his dear mother.
3) The red fingernail provides excellent misdirection and is just plain cool.

I think that is the answer. Someone told him it would look cool.

Answer 2

It got your attention, didn't it?