Peter Sykes mentions in Guidebook to Mechanism in Organic Chemistry (6th ed.) on p.132 that there would significant differences in physical properties of the resultant mixture when we:
- React methylbenzene with $\ce {HCl}$ to form a $\sigma$ complex
- React methylbenzene with $\ce {HCl}$, with $\ce {AlCl3}$ as Lewis acid catalyst, to form a $\pi$ complex
One of the differences he mentioned was that the former would remain colourless while the latter would give a green colour. However, he did not make clear what gives rise to the colour difference. Is it due to formation of the $\pi$ complex?