First off, did you get a TDS meter with the RO/DI unit? Its impossible to know "normal" readings without it.
Color changing DI resin is a very poor indicator of resin condition, again you need a TDS meter, its the only reliable method of knowing. Colors may chage all at once, it streaks, fro the bottom up, top down, not at all or any number of ways and normally by the time it has changed its too late and you are passing nitrates, silicates and phosphates into your tank. Again get a meter and use it every time you make water.
As for normal before the DI, it is entirely dependent on the quality of your RO membrane. If you got the 75 GPD as you should have it should be 96 to 98% less than what is coming out of the tap at that point, but again without a TDS meter you will never know.
Test your tap water TDS, RO only TDS and finally RO/DI TDS to determine how ell it is all working and the condition of your RO membrane and your DI resin.
By the way you purchased a very good RO/DI system!