There isn’t a single “cheapest” carrier for Colombo (CMB) → Dubai (DXB) — lowest fares change by travel dates and sales. However, on recent searches the lowest fares are most often found on the low‑cost carriers:

• Air Arabia — frequently shows the lowest round‑trip/one‑way fares. (Google.com)
• Flydubai — also commonly among the cheapest direct options. (Google.com)
• FitsAir (a regional low‑cost operator) appears in many fare searches as the cheapest on some dates. (Trip.com)

Full-service carriers (Emirates, SriLankan) sometimes match or beat low‑cost fares during promotions, but generally cost more. (Google.com)

If you want the absolute cheapest price for your trip, tell me your travel dates (or allow me to search flexible dates) and I’ll compare current fares across those carriers and major search engines.