Following on from the post below, here are a few other suggestions for a hint of red in salads...
Above: The far red plant is Red Orach, an impressive annual that can grow up to 6ft high! Use young leaves for salads. They have a spinach-like flavour and silky texture.
The near red plant above is Perilla. An annual herb that is part of the mint family. I grow a variety called Shiso and the leaves look a little like stinging nettles. The flavour could be compared to mint or fennel.
I can also recommend the fiery purpled-leafed mustards such as Red Giant. They add a bit of a kick and vibrance to salads. Pick the leaves young, as the peppery flavour intensifies as the plant begins to run to seed and can make your eyes water! They are slower growing than the other oriental brassicas but I also use them as a cut-and-come-again crop.