Tipping Sri Lankan hairdressers is really a play-it-by-ear affair.
The question starts with what type, or should we say, level, of salon you are in. If you are in a chic salon, you will find yourself paying prices bordering on those in western countries.
You would certainly expect well-trained, professional staff at your disposal, who are consequently relatively well-paid and in little need of your tip. If you find yourself in a more common establishment, first ask yourself whether or not you are happy with the hygiene there, and then proceed to calculate a small gratuity, perhaps RS 100, for the boy or girl shampooing your hair and something larger for the hairdresser.
Iif you are satisfied with the outcome, then 10 or 15% of the bill should be more than enough.