Manipulating transparency with mobile shader (particle/VertexLit Blended) works on computer, but not on device

Basically I have a background plane of "stars" that should fade as the player gets near to a planet. Here's the relevant code as it is now:

 GameObject starFieldObj = GameObject.FindGameObjectWithTag ("Starfield");
 Mesh mesh = starFieldObj.GetComponent <MeshFilter> ().mesh;
 Color [] starField = mesh.colors;
 
 void SetStarfieldAlpha ()
 {
     // The alpha we will eventually set the starField to
     float totAlph = 1.0f;
         
     // For each planet of this controller...
     for (int i = 0; i < planets.Length; i++)
     {
         // If the alpha for this planet is less that the previous smallest alpha...
         if (planets [i].ReturnStarfieldAlph () < totAlph)
         {
             // Make it the new reference alpha
             totAlph = planets [i].ReturnStarfieldAlph ();
         }
     }
         
     // For each vertex color...
     for (var i = 0; i < starField.Length ; i++)
     {    
         // Set the new opacity
         starField [i] = new Color (starField [i].r, starField [i].g, starField [i].b, totAlph);
     }
         
     // Set the new color for the mesh
     mesh.colors = starField;
 }

It works perfectly as intended on my computer, but when I build it on an actual device (in this case an iPad 2) the starField doesn't fade at all. Anyone know what I'm doing wrong?