Inline TDS meters lack ATC temperature compensation so can be significantly off if your water temperature and air temperature(which is what these meters sense, not water as they should). I have two dual in lines which I never even turn on anymore as they are never correct. On the other hand a good handheld such as HM Digitals YDS-3, TDS-4TM or AT-1 are temperature compensated and do monitor the waters temerature when you insert the meter. Much more accurate and versatile.
Color changing resins actually contain dyes which dissolve in the treated water, adding TDS by the way.
Level isn't really a big deal since the water is pressurized through the sediment filter, carbon block and the RO membrane. If you had a horizontal DI, which you do not, level is more important since the water follows the path of least resistance. With vertical DI it isn't critical as the water should be filling from the bottom and exiting the top so all wate rand resin will come into contact with each other.