Well, weed can be laced with cheaper chemicals that enhance the effect making the buyer think that they are gettin super great quality weed becuase it makes them freak out and get hyper and shit. Just to let you know, I have seen purple strains, obviously purple haze.
Without getting into the whole purple haze going extinct in the 80s controversy or fact that 'Haze' strains have a low yield and very long flowering times which make it a bitch to grow for anyone with just cash on their minds, i'll tell you that other strains of marijuana can be purple due to the plants genetics or temperature where it is grown. I could get into chlorophyll, carotene, anthocyanins, and xanthophyll but I doubt many care and it really isn't necessary.
Anybody smoking purple haze obviously must either know someone who grows it or be in an area where only the best quality of cannabis is sold, assuming this is the case why would you need to worry about your buds being laced?

I am also very interested in how all these colours are being formed by these chemicals, for instance
Ok if you get bud with white powder on it wit brown spot its been laced with crack
Where is the brown comming from? I've voluntarily rubbed crack on cannabis or put rocks on top of packed bowls and never once did a brown spot appear. Then comes the whole logical flaw again, crack is fucking expensive. You would know if you smoked any significant amount of crack cocaine, it has a distinct taste and it would numb your mouth and throat. If you honestly believe your shit is laced with crack, e-mail me and we can arrange for a sample to be sent to my 'laboratory' (pipe) and 'examined' (smoked).
If you want to talk about substance detection by taste, PCP is described as having a very chemical taste and 5meo-dmt tastes like plastic.

LSD by the way is colourless, you would not know if there was any on your herb even after you smoked it because I second here what thcbongman stated, acid is destroyed at such temperatures rendering this method of administration a waste. Not to mention the fact that plenty of people are willing to buy acid alone, why complicate and waste things by instead secretly putting it on somebodies pot?