They compare the colors of the spray with the colors of your cans.. If enough of the colors are the same that would be a pretty tough evidence againt you.
They do this all the time..
Here's a bit of reading for the interested: http://www.ci.minneapolis.mn.us/police/crime-prevention/docs/graffitiremoveltechniques.pdf#search=%22iron%20chloride%20etching%20metal%20graffiti%22 Know your enemy!
It looks like you have way too much thinner in it.. Acetone, Alcohol and paint thinner has the same thinning effect. brake fluid is also thinning your mix down..