iPad per-vertex color doesn't always work

I'm trying to draw large number (thousands) of particles with per-vertex color using GL class like this:

 void OnPostRender()
 {
 ...
 GL.PushMatrix();
 material.SetPass(0);        
 GL.LoadPixelMatrix();
 GL.Begin();
 GL.Color(...);
 GL.TexCoord2(...);
 GL.Vertex3();
 ... etc. etc.
 GL.End();
 GL.PopMatrix();
 }

I've got a shader that look like this:

 ...    
 Blend One One
 BindChannels {
    Bind "Vertex", vertex
    Bind "texcoord", texcoord
    Bind "Color", color
 } 
     
 SubShader {
     Pass {
     Cull Off    
     ZTest Always                    
     Lighting Off
     ZWrite Off
     SetTexture [_MainTex] {
     combine texture * primary
 }
 }
 }

It works perfectly on PC and Mac, but doesn't draw anything on iPad (primary seems to be (0,0,0,0)) in case I don't have any OBJECTS WITH TRAILS rendered right now. Basically - the moment any trail renderer appear on screen - my particles appear too.