Look on top right of your blender viewport for shading icons:
Click the material preview option:
Some material should appear on your loaded model:
Select any of the layers of your model:
Once selected go to the material tab:
You will see a list of materials. Each one corresponds to a material in your GCode. If you want to edit the color. Just click one material. The click Colour under the Surface section and pick whatever color you like:
Congratulations. You now have a multi-material model that you can render:
