You definitely can see a tint in M color diamonds. The higher the color, the brighter a diamond will appear, assuming they are well cut. In a modern cut stone, I'd want no lower than I color for an engagement ring, but I know people who did go with J color in superideal cut stones (such as from Whiteflash.com) and were happy. However, I do have a couple of right hand rings with very low color light yellow antique cut diamonds set with yellow gold around them to enhance the color (and white diamond halos for contrast).