Why only red channel works in fragment shader?

Question

I'm using QOpengGLWidget to render a defaultFrameBufferObject,This is my fragment shader:

#version 430 core

layout(location = 2) uniform sampler2D texDisplay;

in vec2 texVert;
out vec4 color;

void main(){
    float co = texture2D(texDisplay, texVert).r;
    vec3 cube= vec3(1.0f,0.0f,1.0f);
    cube *= co;
    color = vec4(cube,1.0f);
}

When cube set the value to vec3(1.0f, 0.0f,0.0f) / vec3(1.0f, 1.0f,0.0f) / vec3(1.0f, 0.0f,1.0f), all these values work well that there will be the corresponding color. If cube is set to vec3(0.0f,1.0f,1.0f) or value in which the red channel of the cube is zero, I just get a black window, why does this happen?