I took a look at the great coffee cup in the scene folder of Keyshot 11. The inner surface has been colored via 3D Paint. The transition to the black base color is perfectly straight (see attachment). In Photoshop you can use the Shift key to create a straight freehand line. Unfortunately this does not work in Keyshot. Does anyone know how to create such straight lines with the freehand brush in keyshot?
If you look closely, you'll see that it isn't a perfectly straight line. (see attachment). However to achieve this look, you don't need to use 3D paint.
Try this and you will get a perfectly straight line.