I have chosen a colour in the ‘Colour Wheel’ and successfully applied it to a 2d face. Later when I want to use the same colour on another 2d face the eyedropper tool doesn’t seem to pick up the same colour. Similar but not the same.
Do the faces have different orientations relative to the camera? If so, what you are seeing is due to the shading SketchUp applies to create a 3D appearance. You can overcome that shading if you want by turning on Use Sun for Shading and setting the light slider to the left and dark slider to the right in Shadows.
If it’s all 2D, it’s unlikely to be OpenGL shading. Is it possible you sampled a different area. Do you see two different browns in the Colors in Model collection?
Although I don’t know the MacOS version of SketchUp’s color chooser, I would want to take a look in the list of materials in the model:
When choosing a color for the first time and applying it to an entity, a material object should be created in the materials list. When you want to reuse it, you can just select the material instead of using the eyedropper again. This avoids the possibility that the eye dropper might sample colors as they are displayed on the screen (including shading) and generates a new color that is slightly color-shifted.
On Mac, did you use the command key with the paint bucket tool active, or did you click the little eyedropper on the materials window? The former samples a material. The latter is a pixel color selector that just reads the color of the pixel under the cursor, it does not sample a SketchUp material.