You could say the same with locomotive numbers, as some of the locomotive numbers that appear in the services are incorrect for the model/livery they are on. However there is a compromise. I feel in many instances that the compromise would be worth it for the additional liveries, as seeing AI trains coming the other way in different liveries adds to the variety and interest for the majority customers, whilst only detracting from the realism for a tiny minority of customers. The Class 37 in TVL could have come with a coal sector version, the base livery is identical, no changes would be required to the model and it would compliment the HEAs on the route. As for testing, once the majority of the testing is complete, and provided there are robust development processes in place, adding additional liveries shouldn't increase the testing load by much. The robust processes however are extremely important as adding a livery only becomes an issue when there are lax processes in place and as a result perhaps one livery ends up with old model or physics files.