I don’t yet have any tutorials showing how to paint objects in seamless 3 but I very much hope this will soon change. I have the latest version just ready to upload, I was hoping to upload it before the new year but I am finding it hard right now just to get a few hours to document the changes before I upload it, so I may go a few days into the new year but this delay should be a matter of days not months.
A quick set of instructions for how to colour objects:
Put seamless in colour mode by clicking on the paint mode button (located in the top bar). This button looks like a tube of paint.
Click on the brush button to use the paint brush tool.
Colour the geometry with the brush by dragging the mouse over the object.
Select the colours using the colour picker.
You can adjust the radius of the brush (as indicated by the circle around the mouse pointer) by holding down the control key while moving the mouse wheel.
You may need to zoom out to be able to see the circle if you are up close to the object and your brush is set to a big radius.
Note: Seamless will only paint the part that is selected in the scene tree window but this should not be an issue unless you have more than one part node that can be selected.