Plane rendered into rtt with custom shader doesn't show up in GUI.Label

Unity Engine
1

Hello!

I put a material with my shader on a plane, render it into render texture with separate camera.
If I use this rtt to texture another plane - then everything is ok.
But when I try to place it into GUI.Label() I see nothing on the screen.

Do I miss something? )

thanks

Shader "HSB_HSV_Colorpicker" {
    Properties {
      _MainTex ("Texture", 2D) = "white" {}
      _HueShift("HueShift", Float) = 0
    }
    SubShader {
      Tags { "RenderType" = "Opaque" }
      CGPROGRAM
      #pragma surface surf Lambert
      #pragma target 3.0
         
        float3 rgb_to_hsv_no_clip(float3 RGB)
        {
                float3 HSV;
           
         float minChannel, maxChannel;
         if (RGB.x > RGB.y) {
          maxChannel = RGB.x;
          minChannel = RGB.y;
         }
         else {
          maxChannel = RGB.y;
          minChannel = RGB.x;
         }
         
         if (RGB.z > maxChannel) maxChannel = RGB.z;
         if (RGB.z < minChannel) minChannel = RGB.z;
           
                HSV.xy = 0;
                HSV.z = maxChannel;
                float delta = maxChannel - minChannel;             //Delta RGB value
                if (delta != 0) {                    // If gray, leave H  S at zero
                   HSV.y = delta / HSV.z;
                   float3 delRGB;
                   delRGB = (HSV.zzz - RGB + 3*delta) / (6.0*delta);
                   if      ( RGB.x == HSV.z ) HSV.x = delRGB.z - delRGB.y;
                   else if ( RGB.y == HSV.z ) HSV.x = ( 1.0/3.0) + delRGB.x - delRGB.z;
                   else if ( RGB.z == HSV.z ) HSV.x = ( 2.0/3.0) + delRGB.y - delRGB.x;
                }
                return (HSV);
        }

        float3 hsv_to_rgb(float3 HSV)
        {
                float3 RGB = HSV.z;
           
                   float var_h = HSV.x * 6;
                   float var_i = floor(var_h);   // Or ... var_i = floor( var_h )
                   float var_1 = HSV.z * (1.0 - HSV.y);
                   float var_2 = HSV.z * (1.0 - HSV.y * (var_h-var_i));
                   float var_3 = HSV.z * (1.0 - HSV.y * (1-(var_h-var_i)));
                   if      (var_i == 0) { RGB = float3(HSV.z, var_3, var_1); }
                   else if (var_i == 1) { RGB = float3(var_2, HSV.z, var_1); }
                   else if (var_i == 2) { RGB = float3(var_1, HSV.z, var_3); }
                   else if (var_i == 3) { RGB = float3(var_1, var_2, HSV.z); }
                   else if (var_i == 4) { RGB = float3(var_3, var_1, HSV.z); }
                   else                 { RGB = float3(HSV.z, var_1, var_2); }
           
           return (RGB);
        }

      struct Input {
          float2 uv_MainTex;
      };
     
      sampler2D _MainTex;
      float _HueShift;
     
      void surf (Input IN, inout SurfaceOutput o) {
          o.Albedo = tex2D (_MainTex, IN.uv_MainTex).rgb;
         
          float3 hsv = rgb_to_hsv_no_clip(o.Albedo.xyz);
          hsv.x+=_HueShift;
                 
          if ( hsv.x > 1.0 ) { hsv.x -= 1.0; }
          o.Albedo = half3(hsv_to_rgb(hsv));

      }
     
      ENDCG
    }
    Fallback "Diffuse"
  }
2

Solved. The issue was with alpha channel in RTT.