Those only test for the presence of any tryptamine, not LSD specifically. So even if it tests positive you could still be getting DOM/DOB/DOI. Which is bad, since those can kill you or else have serious negative effects if you take too much.
DOM, DOB and DOI are not tryptamines, but rather psychedelic amphetamines. This is an important distinction, as it means that the most common test kit (Ehrlich reagent), will not confuse them for LSD, contrary to what you said.
It is however true that there are substances which both fit on a blotter, and are indole psychedelics - e.g. 5-MeO-AMT, or other (other than LSD, that is) ergolines, such as LSB, ALD-52 (though these are extremely rare and (likely) share the safety profile of LSD). The Ehrlich test is not capable of distinguishing between different indoles, so you may get a case of mistaken identity in this scenario.
I'm having trouble finding in-depth information about what a Modified Ehrlich's Reagent LSD test actually tests for. Would you mind telling me where you learned they test for any tryptamine, and any more information you might have on the matter?
I believe Ehrlich's tests for indoles, a family which includes tryptamines, but also lots of other things. Colour tests are never conclusive; increasingly persuasive results are usually obtained by combining several different colour tests, for example this post discussing LSD testing: http://www.drugs-forum.com/forum/showpost.php?p=1101068&....