You cannot paint trimesh objects - including trimesh shapes under Gizmos - the way that you can paint primitive objects (shapes in the main asset library). You need to use an external editor (Blender, Adobe Substance Painter, etc.) in order to texture your objects and then import the object as an FBX and the texture as a PNG using the naming conventions provided in the documentation.
You CAN, however, tint your trimesh objects. This will not produce highly saturated coloration, but it creates what I would describe as a color overlay on the object, and you can adjust the darkness/brightness. It truly is a tint vs painting an object entirely. Think of staining wood vs. painting wood. This is more like a stain.
To do this, go to the Attributes tab under the object properties and change the color values in the R, G, and B boxes. Note that clicking into the white box next to the value boxes will NOT open a color palette. You have to manually enter the values, with each max value being 1. So you will be entering decimals for each color. Takes a little trial and error but once you get the hang of it, you can achieve the color overlays that you are looking for.
Hope this helps.