Please give us the option to change the viewport background, grid and brush colors.
Thank you for your request. I have entered a feature request, UE-17080, to be considered by the development staff.
I have some good news. It turns out the grid change is already available in the editor. If you enable your “show engine content” checkbox in the content browser and type grid into the searchbard, you should get to the material used in the grid. It turns out the grid is a math-generated material and can be changed through the material itself. It is not highly recommended to do so, however, and I would try this in a backup project first to prevent damage to your original project.