axmol/cocos/renderer/shaders/ui_Gray.frag

const char* grayScale_frag = R"(

#ifdef GL_ES
precision mediump float;
#endif

varying vec4 v_fragmentColor;
varying vec2 v_texCoord;

uniform sampler2D u_texture;

void main(void)
{
    vec4 c = texture2D(u_texture, v_texCoord);
     c = v_fragmentColor * c;
    gl_FragColor.xyz = vec3(0.2126*c.r + 0.7152*c.g + 0.0722*c.b);
    gl_FragColor.w = c.w;
}
)";