raylib/examples/resources/shaders/shapes_grayscale.fs

#version 110

uniform sampler2D texture0;
varying vec2 fragTexCoord;
varying vec4 fragTintColor;

void main()
{
    vec4 base = texture2D(texture0, fragTexCoord)*fragTintColor;

    // Convert to grayscale using NTSC conversion weights
    float gray = dot(base.rgb, vec3(0.299, 0.587, 0.114));
    
    gl_FragColor = vec4(gray, gray, gray, base.a);
}