Short answer: Most of the flight search sites show IndiGo as the cheapest carrier on the Colombo (CMB) — Chennai (MAA) route (with Air India and SriLankan as other nonstop options). (google.com)

Notes and recommendations:

• Airline fares change by date, time and how far ahead you book, so the absolute cheapest airline for your trip depends on your travel dates. Recent searches (Nov–Dec 2025 windows) show IndiGo offering the lowest fares. (google.com)
• If you want the cheapest available fare for specific dates right now, search and compare Google Flights, Skyscanner or a metasearch (Trip.com / Wego) and include +/- 3 days to catch the lowest-priced day. Example search results and price comparisons show IndiGo as the lowest on this route. (google.com)

If you’d like, tell me your travel dates and whether you prefer nonstop vs. connecting flights and I’ll look up current lowest fares and give specific options. (I can search for real-time prices now.)