Recently I have repainted a class 387 and put a livery on it but at the front of the train you can’t paint the middle bit where the wind screen is
Unfortunately, it has been like this since it was released in October of 2021. I got around it by making the base colour of the train the colour of the front doors, but if you wanted multiple colours there I'm not sure how to do that.
The "Middle bit" Has a name. It's called gangway doors. Yes I've had this problem many times It's super annoying and I doubt that it will ever be fixed.