My resin turns brown prematurely as well (it's been brown for 3 months and my TDS is still 0). Never found out exactly why. I E-mailed BRS and got the reply below. If you have a TDS meter and it's reading 0 after the DI canister, it doesn't matter what color the resin is - it's working fine. (remember the color is meant as an
indicator of function, the resin doesn't depend on the color to do its job.
Some peoples water supply can make the Di resin Change color faster than
some other water supplies. Really when it changes color it is just a good
time to start watching your TDS meter. There are a lot of things that can
cause the DI resin to change color quick. The most common of which is high
CO2 in your water source. If that's the case there are technically things
you can do to mitigate that but in most situations its just more cost
effective to buy more DI resin.