Havret's fellow Frenchman Jean-Francois Remesy posted a 67 to join Spain's Ivo Giner, Garry Houston of Wales and Spain's Emanuele Canonica in a tie for third at 5 under par.
Colsaerts, who made his first cut of the season two weeks ago at the Open de Portugal, played the back side first at a rainy Real Club de Golf de Sevilla and opened with birdies on each of his first two holes.
The 21-year-old added back-to-back birdies from the 13th before dropping a shot with a bogey at the 15th. Colsaerts then mixed a birdie and two bogeys over his next three holes to make the turn at minus-2.
Colsaerts made his move on the front half and ran off five birdies over a stretch of nine holes to stand alone atop a tournament on the European Tour for the first time since he led after the first round of the 2003 Open de France.
Havret, who won the Italian Open in 2001, jumped out of the gate with a birdie at the first but gave that shot back with a bogey at the eighth.
'After I won in 2001 I don't think I did the right things, especially in the winter following, and had a bad 2002,' said Havret. 'But now I've changed my winter training and changed my coach, things are looking up again.'
He countered on the inward nine, however, and birdied four in a row starting at the par-4 10th.
Havret then birdied the 15th and the 16th to finish one shot off the lead.
Miguel Angel Jimenez, who is looking for his third win of the season, started on the 10th and collected three birdies and a bogey over his first nine holes to make the turn at 2 under.
Jimenez failed to reach the green at the par-5 fifth but chipped in from off the green for an eagle.
The Spaniard found trouble with a bogey at the eighth and hit a second shot to the par-5 ninth that almost led to disaster.
Jimenez's approach to the ninth bounced in the fairway and carried over the water that protects the right corner of the green. His ball came to rest on the back edge of the putting surface and Jimenez took two putts for a closing birdie.
Jimenez was joined by Gary Evans, Nick Dougherty, David Lynn, Johan Kok, Wayne Westner, Steven O'Hara, Cesar Monasterio, Gabriel Canizares and Raul Ballesteros in a tie for seventh at 4-under-par 68.