I'm helping out with one of those crowdsourcing transcription projects and I've come across a chap from Kentucky called Thomas Stanton, who joined the 15th Regiment, US Colored Infantry.
His complexion is listed as "Yellow", which I had imagined would refer to an East Asian origin, but his name suggests otherwise. A white man with jaundice wouldn't be joining the Colored Infantry, presumably?
Since you are working with enlistment papers and not commission papers, that indicates Thomas Stanton is joining the regiment as a private, not as an officer.
Colored regiments did have white officers in command, but as a private, Thomas Stanton was clearly considered to be an African American man. With that information in mind, the note about his complexion being "yellow" is related to the term high yellow to refer to an African American with light skin. Mr Stanton probably had more Caucasian ancestors than African ancestors, but at the time the idea was enshrined in law that any person with any African ancestry was categorized as "colored".