Assuming that you're referring to the Colt Chrono Automatic and not the Chrono Colt Quartz, this is an old watch made during 1997-2000 so will be cheaper than most Chronomats which will be newer. Also, the Colt was not COSC certified whereas post 2000 Chronomats are.
The differences between a Colt and a Chronomat from the same year are small. The cases are different. The Colt is marginally bigger/heavier. The Colt has mineral glass whereas the Chronomat has sapphire. The Chronomat dial has applied markers. I think it's simply that the Chronomat has always been Breitling's flagship model so commands a premium price. Head only prices in 1999 were £1145 for the Colt and £1455 for the Chronomat.
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