Stouts are defined by their roasted grains, and yeast. If you have those taken care of, you will have a great stout.
You can always add a splash of crystal to get a bit of color and flavor back if you're really worried
If you are subbing for Maris Otter, consider going with the briess pale ale malt vs. briess 2-row brewers malt--they have some similar characteristics. Keep in mind that even more than color, fermentability and extract will vary between MO and Pale Ale malt. Check the spec sheet or plug it into some software if you haven't already.