This is a tool for programmers — It does not convert images.
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.
To switch between the shaders, tap anywhere in the top-right or top-left of the image to move forward/back in the list. Tap anywhere on the bottom of the image to toggle between the full-color image and the currently-selected greyscale image. Toggling between the color and greyscale versions of an image allows the user to carefully inspect the results of each shader.
See Support URL for full details about the app including the source code and shaders used.