Colorado is "best interest of the child standard" technically, but "fathers' rights activists" have tried to pass 50/50 legislation on a number of occasions, and as a result, most judges and lawyers default to 50/50. Most parents assume it is 50/50. So in cases where the parents get along, the parents work out what's best for the kids. In cases where there is conflict or abuse, dad will almost always go for the full 50% to minimize child support and stay in control of the family. No distinction is typically made whether the parents are married or unmarried. Sole custody is extremely rare unless one of the parents has been convicted of physical abuse or neglect of the kids.
Just out of curiosity, which states are you referring to where unmarried mothers typically get full custody?
ETA: I'm not an attorney--sorry!--but I am a social worker in the DV field.