I've never seen a yellow purpurea so the question hasn't arisen for me. The leaves are a slightly different shape and in luteola they are bright, Granny Smith apple green whereas in purpurea, the white and pink anyway, they have a more downy look and are a little larger. The flowers are larger too. There is an attractive form of luteola called var. maculata which is heavily blotched with a beetroot red colour in the leaves.
A friend, no longer alive, always managed to have luteola, the white purpurea and massoniana in flower at the same time in her garden, the three growing together and the colours looked amazing in a patch. My massoniana is just about over when the other two start.
