Area from Distance Color Maps

I am studying the instability of the medial cuneiform - 1st metatarsal joint. I assume that the group of cases with arthrosis has a different joint space than the control group.
For this, I successfully created a model of the medial cuneiform with the color map of the distance to the articular surface of the 1st metatarsal (first image).
My question is: Is there a way to extract the area that corresponds to a specific color (for example, the color green in the second image)?

CM - I meta anterior


*Image subtitle:

  • Navicular: blue;
  • Intermediate cuneiform: red;
  • Medial cuneiform: rainbow;
  • Base of the first metatarsal: yellow.