it has been ages since my monkey has smoked the deems (only aya ocassionally these days for deem consumption) but when it did it prefered the oxidized version. aka clear crystals that turn tan-brownish looking after heat-cold cycling. it was a slower onset which it prefered but required more substance. it forgets, the high may have lasted longer as well.
spice can also be brown due to organic contaminants but you mentioned it turned this way over time.
all of my posts are fictional. please interpret them as such.