According to Oricon's sales data:
Colours: 53,406
A ONE: 52,816
The difference is only 590 copies, but A ONE would have to be able to chart for another 2~3 weeks to outsell Colours.
Of course, well never know how many copies Colours actually sold after it fell out of Oricon's top 300 (as well as how many copies A ONE sold during the weeks it didn't chart), but officially speaking according to Oricon it's possible