As you can see in the image, I have selected the whole model, pressed 2 to color it with my red PLA filament, but the model remains grey (PETG) and for some reason a purge tower is generated. The other side of the model is also entirely grey, there is not one single red pixel on the entire thing.
That is a bit weird. Does it also act in this way if you right click your object and then select “Change filament”?
And which Studio version are you using? The latest release or the 2.2 Beta?
I have come across this on a few occasions and while I’m certainly no g-code or 3d modeling expert I started trying different things to get rid of the extra colour like you are experiencing. In the end I right clicked the model and selected “fix model”. So far in every instance of this problem this has worked for me.