Now try this out. It's not a grayscale shader like some people thought - it's basically a "moody shader".
gl_FragColor.r = gl_FragColor.r * 0.33 + gl_FragColor.g * 0.33 + gl_FragColor.b * 0.33;
gl_FragColor.g = gl_FragColor.r * 0.33 + gl_FragColor.g * 0.33 + gl_FragColor.b * 0.33;
gl_FragColor.b = gl_FragColor.r * 0.33 + gl_FragColor.g * 0.33 + gl_FragColor.b * 0.33;