Material SetColor in a loop not working on Android

Ok, so I'm being blinded by something obvious here, so I'm really hoping a fresh pair of eyes can help me out!

I have a custom shader with a _ColourBoost Color property as follows:

 Shader "CameraFeed" {
     Properties {
         ...
         _ColourBoost ("Colour Boost", Color) = (0.2,0.4,0.2,1.0)
         ...

I can set that property in the material from a C# script, and it works as you'd expect when run in both the editor and on the Android device I'm targetting:

 CameraMaterial.SetColor("_ColourBoost", new Color(0.39F,0.5F,0.39F));

But, if i try to update that value in a loop (e.g. a fade starting from black and ending at the chosen colour), something like this:

 private float progress = 0.0f;
 while(progress<1) {
   CameraMaterial.SetColor("_ColourBoost", new Color(0.39F*progress,0.5F*progress,0.39F*progress));
   progress += Time.deltaTime/6; // Fade in over 6 seconds
 }

If I play in the editor, it appears as I expect - fading the colour in from black over 6 seconds. But when deployed to my Android device, it apparently does nothing - the material doesn't change and remains exactly the same over the course of the loop.

If it makes a difference, the loop is contained in an OnGUI function, but what I really don't understand is why the behaviour in the editor doesn't match that on the device. Any ideas what I'm missing here? Thanks!