Z60t titanium lid changing color

[wujstefan]

Hello,

I just put some love to my Z60t's with titanium lids and started cleaning the lids. The problem is, while cleaning one of them with alcohol to get rid of some black pen marks I found out that the lid is turning way brighter.

Kind of metallurgy question since I have no idea what kind of cast is that - am I damaging the outer protective layer of the lid, of is this just "really that clean"? Should I use concentrated alcohol for such tasks? I am pretty sure when used on plastic parts those would loose their colour.