Odd color reaction with Marquis and MDMA

The textbook colour reaction of Marquis reagent (mixture cc. $\ce{H2SO4}$ + formaldehyde, optionally with some methanol to slow down polymerization) with MDMA (3,4-methylenedioxy-methamphetamine, the party drug "ecstasy") is a gradually appearing dark purple/blue colour.


Recently three different samples were examined with marquis. The samples were taken in different countries, at different times. They looked a bit different too, one was light brown, the other two were clear.


TLC results suggested they are the same compound with $R\_f$ values that of MDMA.


Reaction with Mecke reagent was also MDMA-like.


Reaction with Marquis started OK, with fizzing (a sign of a HCL salt) a light and then dark purple colour appeared.


After maybe a minute or two, the dark colour started to fade, it turned into light purple. As the colour faded even more, it turned green, and then to light yellow.


The test was repeated with two different reagents and all three samples, the results were the same.


I found no documented nor anecdotal record of marquis turning green/yellow in the presence of MDMA after a few minutes.


* Is this normal? Should Marquis turn green/yellow after a few minutes in the presence of MDMA?
* If this is really unusual, then what could possible cause fading & color change?


