I just colored my hair but it didn't cover my gray roots when can I color it again with a darker shade

If you're going darker, then you can do it right away. Is this a box color from the store? Or is it from a beauty supply store, where you have to mix the developer along with color?

If option B, then make sure you are using 20V developer with the correct amount of time for great coverage. Perhaps you allow some to apply to your hair for equal coverage. Hope that helps some.