Amirthaa is a cheap, clean, pure-veg South Indian restaurant in Bambalapitiya that serves a good thali and various fried South Indian and Sri Lankan snacks such as thosai, paratha, samosa, and wade. As far as I'm aware it's part of a chain, or one of a couple of Amirthaa hotels in Colombo (though it could be another symptom of something I call 'Pilawoos Syndrome', a phenomenon common to Sri Lanka, where restaurants with the same or similar names but a little or no connection to each other multiply almost infinitely across a small area). This branch (or not, as the case may be) of the restaurant is really good for the price, and it stands out among the numerous, mostly average, eating joints along this stretch of Galle Road.
I went for lunch there last week and ordered the thali. This consisted of rice (obviously), bean curry, dhal, potato curry, chutney, papadam, wade, curd and a sweet. The whole meal was pretty mild spice-wise but very tasty nonetheless, and the whole thing only left a dent in my wallet of 140Rs which isn't bad at all considering the variety and size of the meal. The aloo paratha
that my friend ordered was a little on the greasy and heavy side but tasted damn good nonetheless. The paratha cost around 60Rs.
It's more of a quick-bite sort of place than a sit-down-and-chill restaurant, and reminded me of a lot of the lunch places I dined at in South India with it's strip-lights, white tiles, and the token 'WELCOME' spelled out in fake red and yellow flowers above the entrance. As mentioned though, it's clean, and for what it is it definitely does the job and more.
As Indi and Shru have both commented elsewhere on this website, South Indian food doesn't seem to be as popular as the more meat-heavy cuisines on offer here in Lanka, but when veggie food is as delicious and satisfying as this it's sometimes hard to see why that's the case.