What happens when I use cyclohexanol to prepare cyclohexene?
The acid-catalyzed dehydration of alcohols involves a carbocation intermediate, which can act as a Brønsted acid; the loss of a proton yields the desired cyclohexene. Furthermore, the carbocation may undergo other side reactions. In particular, the carbocation may be subject to a reaction with other molecules to give substitution products; or the carbocation may undergo carbocation rearrangement.

The coloured residue of the distillation is probably the result of cationic polymerization of cyclohexene. This reaction in combination with rearrangements may yield various oligomers.