Yes, I have tested many pressed pills that had only a purple result.
You ought to browse the results here:
DrugsData.org lab analysis and drug checking results
www.ecstasydata.org
Drugs Data/Ecstasy Data tests all kinds of drugs, not just ecstasy, you you may need to scroll quite a bit to see results for ecstasy pills.
MDMA only pills are quite common. That is what ecstasy is supposed to be. Anything other than MDMA is NOT ecstasy, it is an adulterant that should not be there.
It would be like if you went out and bought Tylenol. It should contain acetaminophen. If it contained something else, well, it would not be Tylenol.
Yes, you are correct that an MDMA only pill will only react purple. You can see that in detail when you click on the individual reports, like here:
DrugsData (EcstasyData) lab testing result
www.ecstasydata.org
To further confuse all of this, there are many "copy cat" pills that seek to imitate a known, good pill. So, maybe originally Teslas were only MDMA and had a good reputation, then people started copying them and including adulterants in the pills.