Bud comes in different colours, many strains produce blue-ish purple buds, red, yellow, orange and varying shades of green.
You would know if your marijuana had formaldehyde on it because although it is colorless it SMELLS.
Formaldehyde by the way is not phenylcyclidine (PCP) or vice versa, the mix up comes from its slang terms 'wet' and 'embalming fluid'. http://www.erowid.org/chemicals/pcp/pcp_info6.shtml

The problem I have with this 'lacing' issue is that I cannot think of any logical reason why any real drug dealer would take ordinary cannabis and mix it with a more expensive substance just to sell it to the unsuspecting consumer for the regular price of herb.
Since PCP is cheap, maybe, but it is usually put on shit weed and clearly marketed under its slang 'wet'. Only worry about this if you're dealing with the unscrupulous type, and then perhaps you should find a better dealer.
Nullific Reviewed by Nullific on . Laced or Not? How can you tell if your Bud is laced or not? Rating: 5