Strictly speaking, Chameleon and Invisibility are two different spells. Invisibility made you undetectable until you activated something or attacked or spoke to someone. Chameleon was percentile and, when brought to 100%, had the same effect of invisibility, but also did not end when you attacked or activated something. You could also sneak while under a <100 Chameleon effect to make yourself harder to spot.

You could get an amulet with 80% Chameleon if you followed a woman asking for your help in the West Gash Region. There also are some rings to complement it and reach 100% that I believe are random (shadowweave and shadowmask rings).