How to apply specific texture for specific triangle in mesh based on height in shader?

Hi, I've created a terrain and I can paint each individual triangle in the right color based on the height of the middle of its edge.

  public void CalculateColors(){
         float сentre;
         for (int i = 0; i < triangles.Length; i = i + 3){
             сentre = (vertices[i].y+vertices[i + 1].y)/2;
             if (сentre > 100){
                 colors[i] = Color.blue;
                 colors[i + 1] = Color.blue;
                 colors[i + 2] = Color.blue;
             }
         }
     }

but I want to apply texture for each triangle in mehs. I know how to do it for each pixel.

 void surf (Input IN, inout SurfaceOutputStandard o) {
             float heightPercent = inverseLerp(minHeight,maxHeight, IN.worldPos.y);
             float3 blendAxes = abs(IN.worldNormal);
             blendAxes /= blendAxes.x + blendAxes.y + blendAxes.z;
 
             for (int i = 0; i < layerCount; i ++) {
                 float drawStrength = inverseLerp(-baseBlends[i]/2 - epsilon, baseBlends[i]/2, heightPercent - baseStartHeights[i]);
 
                 float3 baseColour = baseColours[i] * baseColourStrength[i];
                 float3 textureColour = triplanar(IN.worldPos, baseTextureScales[i], blendAxes, i) * (1-baseColourStrength[i]);
 
                 o.Albedo = o.Albedo * (1-drawStrength) + (baseColour+textureColour) * drawStrength;
             }
         }

I thought to use colors of mesh as mask for textures, but I'm not sure is it good way or not.

Maybe there is a way to do it right? Thank you for any help in advance.