Hi Thomas, I am confused; in your screenshot you don’t have ‘Tank_Panels_1’ selected in the Shader Editor, so I thought that was it. It is the active texture (see Texture Slots in your Tool Settings and the UV Editor).
As far as I can see, you are trying to paint on the ‘Tank_Cannon’, used the Fill Tool on the ‘Tank_Base-Color_1’ and switched your Texture to ‘Tank_Panels_1’ or something like that…I can’t tell much more from the Screenshot.
But it has absolutely nothing to do with the higher resolution.
Could you send a link to your file? (use Dropbox or any Cloud service..) Make sure to go to File > External Data > Pack All into .blend, so that it includes your image textures.
