Hi
When you change the rim colour you need to check the box to enable the tweak then it's the second of the four boxes where you need to click on and selet the colour you want, I used this to change the Mercedes and Caterham rims - when you have your colour, select apply and save your gp4tweak file
for the wheel positions again check the box to enable the tweak - then enter your settings for each driver's wheels then apply the changes
hope that works ok