The 4-16x50 Viper I had would range find by parallax adjustment very well out to about 45 yards on 16x mag - I'd imagine it would be accurate enough for your purposes out to 30/35 yards on 12x mag.
Temperature sensitivity of the scope only really affects it's parallax adjustment range finding. I'm sure you'll find people who will tell you that when the temperatures hit 34 degrees they noticed that their POI changed and came to the sweeping conclusion that it must
have been a problem with their scope. What they won't have thought about though is the difference in air density, the differences in heat expansion on the different types of metal used in the construction of their rifle and a number of other atmospheric parameters have an effect on exterior ballistics.
There's a slight language barrier you might have already noticed too.. over here FT (Field Target) and HFT (Hunter Field Target) are shot on different courses with different rules. So yes, the SCB reticle was designed with FT in mind, its the HFT version of FT that was in mind, not the FT version of FT ( if that makes sense
Personally, based on your requirements I don't believe you'll see any difference between a black viper and a silver one, but why not drop Gary / Sammie an email at email@example.com
and they'll tell you if a Silver 4-16x50 exists.