Setting a variable to the amount of red pixels in a texture

You can use https://docs.unity3d.com/Documentation/ScriptReference/Texture2D.GetPixels.html

And then check the value of each color. May be a little long though.

 public Texture2D texture;

 int GetRed()
 {
      int _result = 0;
      Color[] _color = texture.GetPixels();
      foreach(Color col in _color){
         if(col == Color.red)_result++;
      }
      return _result;
 }

This will return a value for any pixels fully red, you may want to makes it a little more flexible so that it includes color within a range of red like

 if(col.r > 0.9 && col.g < 0.1 && col.b < 0.1)_result++;

this is regardless the alpha value.