Click on either of these two modes on the top right of your 3D View. The Left one of the two is Material Preview and the right one is Rendered Preview . Explaining each one is not in the scope of this documentation but the left one will generally be less taxing on your computer and faster.
This is what your model will look like after you pick one of these. Both are shown below:
Material Preview
Rendered Preview
I’m showing the rest in Rendered Preview but the process is the same in both of these. Select any layer on your model:
Then if you go to the Materials tab in the Properties window. If you have a multimaterial GCode this tab will look a little different. Take a look at: How to Work With Multi-Material GCodes
Just select the material Stitcher_Material_0 and change the color to what you want to:
It is advisable to keep the colors slightly unsaturated as they look more realistic.
The Gloss Color option will change the color of the shine. It will be advisable to keep it some shade of gray but you can play around for some creative results:
The remaining three options are explained in the UI itself as well. You are not supposed to change Material Roughness a lot but play around with these and figure out some materials you like.