In Colorado. If someone tells me to pawn their stuff, can they charge me with theft

It is certainly possible you could get in trouble for it - even if that seems unfair.  I suppose you could be charged with Theft if the police beleived you did not have authority to take it.  There is also a crime of "False Information to a Pawn Broker" depending on what you said on the paperwork filled out for the pawn broker.  But having the text that apparently says you can pawn the computer to cover part of his rent is important, depending on exactly what it says.