Hi. I could not find the answer in the SketchUp help center, not in YouTube, not in the official SketchUp forums.. Nowhere! But I found it after trial and error. So I'll write it here. Maybe I'll rediscover this post myself some years from now, when I've forgotten about it (has happened before):
In SU for Mac, when you first open a model and want to edit a material, you press B to open the Material Editor. Now, press the "Brick" tab and then the house to see the "Colors in model".
Now click on the material in the model to select it, while pressing Cmd. If this is the first time you're going to edit this material in the current session, the "Colors in model" window will automatically scroll itself so the material you selected is visible... Among 20 other similar ones, and only identified by a faint baby blue outline. Strain your eyes a little bit to find it, and double-click on it to enter the edit mode. Yup. So many steps just to be granted the privilege of editing the material. Now, in the current "brick" tab you can only edit the texture. To edit the color, pick any of the other tabs. If you hit "close" you're out of the edit mode. If you don't, you can keep editing other materials by just Cmd + clicking on the scene.
Now, if you have already edited this material before in the current session, and you have already exited Edit mode, the "colors in model" window will not automatically scroll to it and select it... It will just stay where it is. Yup. It's not gonna help you at all to find the material in the window. It won't even place the faint baby blue halo around it. Weirdly, opening Safari, googling "how to get rid of SketchUp" and then returning does help. Now it does scroll to the material and place the faint, ghostly halo around it, so you can double-click to edit it.
You can also do this: Double click on ANY material in the "colors in model" window to enter Edit mode, and THEN use the eyedropper (Cmd + click) to actually select the material you really want to edit.
I'm still not giving up on SketchUp for Mac.