I realize colors are assigned specific meanings in sketches, so it could be confusing if users had the ability to choose the colors of lines and other sketch objects—to be clear, that's not the feature I'm suggesting.
In some cases, sketches can be complex, and it's difficult to visually identify relevant relationships between the objects. My suggestion is to enable users to apply a *background* highlight to selected objects. The colors of the selected objects would not change, but a line of heavier weight would be visible behind the selected object. I've attached an image to show what I mean. I think a feature like this could be pretty handy.
