Greyscale Image

I have a 2D game that I am porting over to unity. In the game I have a static background image. At the level start the image is converted to greyscale in memory, pixel by pixel. At certain points the color image would appear. To do this in the old game I would make the triangles to the color image and display the triangles on top of the greyscale image.

Any hints on the best way to do a similar thing in unity? I’m very new to unity.

Unity - Scripting API: Texture2D.GetPixels and Unity - Scripting API: Texture2D.SetPixels