This episode is dedicated to the memory of Kelly Lindquist (1958-2005), a costume designer who had died unexpectedly in January.
The mental disability Colin has is never explained in Wishing beyond just some of the suspect characters referring to him as "retarded", "slow" and "special." It's implied that he was born with brain damage rather than any specific disorder, as his mother mentions that Colin was found to be disabled at birth.
Colin Miller died on March 6, 1993.