Colunga sent goalkeeper Marcus Bettinelli the wrong way from the penalty spot just before the hour mark after Kostas Stafylidis upended Elliott Bennett.
And March made sure of a vital three points for the manager-less Championship strugglers at Craven Cottage four minutes from time.
Victory lifts Brighton out of the relegation places at the expense of Millwall on goal difference as the south-coast club battle to safeguard their future in the second tier of English football.
Elsewhere on Monday, Leyton Orient hauled themselves out of the League One drop zone with a 3-0 win at rock-bottom Yeovil Town.
Stephen Arthurworrey had an early effort disallowed for offside for the hosts before Chris Dagnall made it five goals in as many games with a header from Bradley Pritchard's centre.
Kevin Lisbie extended Orient's lead just after the hour to double his account for the League One season, with Dean Cox netting a third for the visitors with five minutes remaining.
The division's other game between Barnsley and Oldham Athletic fell victim of the cold snap in England as the Oakwell pitch was deemed unplayable earlier in the day.
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