When I add the image with a transparent background to the specific face, the appearance of that specific face is slightly darker. How can I fix this?
What happened is this:
1) your whole icosphere is a solid non-transparent white material.
2) The front face is totally transparent (you can see the inside of the ico sphere) except for the colored circle
3) The darker part is actually the shadow inside your ico-sphere
Make your Material.005 non-transparent.
Copy the color from your Material.004, and use your transparent image as a mask. Use Mix shader and connect your alpha channel into fac input.
The primary material uses white Diffuse shader.
The secondary material also uses white Diffuse shader, but masks it with alpha channel of the *.PNG file.
So where the Blender logo has a color, the white Diffuse shader is not applied.
Hope this helps.
If you need any clarification, let me know.