I haven't looked in to it again recently, but a few years back I had a request for strawberry cake. I scoured the internet and was distressed to discover that almost all of them required the addition of red food dye (either directly, or from an added ingredient that has red food dye, ie. strawberry jello). We can't do food dyes in general, but definitely avoid red40 because of allergies.
There were some options out there that were more natural without any added dyes, but for simplicity I ended up making a vanilla cake, with a cream cheese frosting that I added strawberries to to make it "strawberry frosting" and garnished with fresh strawberry slices. It turned out really well.