Hey old_skools, welcome to bluelight. First of all we don't use 'swim' here, you can just say 'I'. Just make sure not to mention any ongoing criminal activity, personal information or other incriminating things and you will be fine.
For MDMA adulterated with meth you would look for an orange color during the reaction with marquis reagent (EZtest ecstasy), before it is overpowered by the dark blue/black MDMA result. I would also not grind the sample to dust but rather breaking it up in tiny pieces, leaving potential little clumps of adulterant together, increasing the likelihood of you noticing the orange color. Marquis normally has no orange whatsoever with pure MDMA. Also make sure you don't use a too big sample as the reaction will be too fast and spread out to notice other colours
Mixtures are difficult to differentiate as you need to watch carefully for any other colors than dark blue/black. These other colours will hopefully present themselves during the colour change, as the final result for MDMA is such a dark colour it will overpower other colours. The change from base colour to the end colour and any abnormal other colours during that change are equally important if not more so than the final colour. Testkits are never 100% foolproof and I think with mixtures they really fall short if you don't monitor the reaction very very carefully and even then it's still not close to 100% foolproof. Far better than nothing of course. if you want to increase the likelihood of spotting adulterants buy a
full testkit with marquis (EZtest ecstasy), mandelin (EZtest ketamine), simons, mecke (EZtest opiates) and perhaps robadope as well. You should at the very least be testing with marquis (EZtest ecstasy) but a full testkit is preferred