Ink shading is something a lot of people look for when they are using a flex nib. Flex nibs tend to highlight shading because of the line variation – the variation in wetness and line size changes the saturation of the ink, resulting in shading. Personally, I love it. I know some people who hate it and want a solid line.
If you are looking for an ink that will shade well with your flex nib, I find that lighter inks will show greater shading, and look beautiful as well.