LittleBear's correct. Pivot pins and trigger/hammer pins might be larger, depending on when the lower was made. I'd take a trip back to the shop and measure the diameter of the pins. Colt's larger pivot pin is .312" (me thinks) and should be a bouble ended screw as opposed to a regular push pin, while "standard" i.e. everyone else on the planet, is .250" push pin. I would definitely measure them though as going by year of manufacture alone might not be reliable. If the front and rear pins are both the same it is a "small pin" lower so you'll be compatible with an other brand upper.
If it has the larger pivot pin, you can get adapter bushings to allow another upper with a .250" pivot pin hole to mate up. If the Colt's lower has the larger trigger and hammer pins, you would need to get parts specific to the Colt's pin dimensions if you wanted to replace the trigger and or hammer.
"People live too long, dogs don't live long enough" - FTF Member-