Had to commit to rename this to what it is.

42-shadow-refactor/shaders/30-screen-bw.frag.glsl

@@ -0,0 +1,13 @@
+#version 330 core
+out vec4 FragColor;
+
+in vec2 TexCoords;
+
+uniform sampler2D screenTexture;
+
+void main()
+{
+    vec3 col = texture(screenTexture, TexCoords).rgb;
+    float grayscale = 0.2126 * col.r + 0.7152 * col.g + 0.0722 * col.b;
+    FragColor = vec4(vec3(grayscale), 1.0);
+}