First off, welcome to OCDO!!!
I would have to say full exposure of the sidearm in its holster. No coverage by a garment.
I've dug and dug and dug for any info on what Colorado defines as "concealed", and for the life of me, haven't found that answer yet. When I CC tho, I wear a tanktop undershirt with my 1911 in a Serpa CQC at about 3:00. My overshirt (tee shirt, polo shirt, sweatshirt) usually leaves the bottom 2 inches of holster exposed. Plus I usually print like crazy. Haven't had a problem yet.
I know that some states go out of their way to define "concealment". Even a slight print can be labeled as "illegal open carry". Gladly, Colorado is not one of them!