I think this is so sweet and I would plan to surprise her with it on a day you two could go out and shop (if that is what she wants) or stay in and talk about it (if that is what she wants). First go out and buy a few cheap things for yourself at the drugstore. Rimmel mechanical pencil eyeliner and maybe some tinted moisturizer and a balm in your favorite flavor. You could get a gift card for Sephora/Ulta ahead of time, too. Throw on the eyeliner in the bathroom to the best of your ability, but save the other two products to hit her with your best eye-liner puppy dog eyes and a, "How do these things work?" Make it fun to learn all her wisdom. Ask her if your eyes are smouldering.

This way you don't have to worry about wearing anything to work, it can be time you just spend with your wife in the privacy of your own home. Ask her to teach you "No make up make up" and she will know what that is. And if she does, you may decide you want to wear that to work after all. If she doesn't feel like shopping don't force it, though. Maybe bring up what your grandmother said and apologize that you think it's made her feel guilty about a hobby she really loves? When I was ill makeup was one of the first things that helped me look in the mirror and see something other than a sick person. It made me happy and boosted my self esteem and at that time it became theoretic as well as a fun, creative outlet. I never left the house so there was nothing vain about it. Best of luck to you and your wife.