I recently released an app that demonstrates various greyscale conversion techniques implemented as GLSL shaders.
Get it for Android and Apple from the respective links on the app promo page:
http://www.roguish.com/blog/?p=775
You can find a link to the 7 shaders and basic Cocos2d-x project on the promo page as well.
Grey Shaders is a simple free app with a very limited scope. If you find it informative and helpful, please give it a 5 star rating to increase its exposure.
App Info:
When converting an image from color to greyscale the perceived brightness of each color is matched with its equivalent greyscale tone. The Grey Shaders app includes 7 GLSL shaders that convert a sample color image to greyscale. Users can toggle between the greyscale versions and the original full-color image for optimal A/B comparison. Toggling between the color and greyscale versions of an image allows the user to carefully inspect the results of each shader.