Thank you for the question.
The sample you helpfully provided shouts "use textures" to me. Three different tactile textures to correspond to the three different printed colors should give the student the best access to the concept being taught.
I fully agree with your reservation about using any general omission symbols. How would you distinguish the pink, green, and blue areas? How many symbols should you use in each area? I think there are no good answers to those questions.
Are you able to use tactile area textures to transcribe this material?