I made white buttercream icing and added baking cocoa and brown food gel it's really light when I whip it, but darkens when it sits. Will t be dark on my cake if I whip if right before I frost the cakes? I need a dark brown and I cannot achieve that color.

Instead of whipping it, just stir it gently and it shouldn't lighten that much.  If you are whipping it then the air is going to make it lighter in color.

I got it brown by adding dark chocolate, cupcake jemma has good recipe for chocolate butter cream and then a little black.