I will certainly not defend the Mac Material interface, it’s convoluted and baffling at times and could really use some UI attention. Editing in an outside program is often a good choice, note that outside editing can be invoked directly from the surface under right click>texture>edit.
However, once the strange logic of how it works is understood, one can do edits to textures from within the Material Picker interface. Colin outlined the correct steps, One must select the texture from the in model list and chose edit via right click or under the “color” menu. Transparency and size are controlled on the first page, switching the color picker to another kind of color choosing interface across the top gives you ways to alter the color.
Note that altering color via the material editor is non destructive and can be changed again, editing outside re-saves the file and is not reversible (it is in the Undo stack, but not after a save).
