I've only just spotted this thread.
I grow a large number of different collections of this species, covering most of its colour range including the yellow, and I find the yellow to be more difficult than the others (less vigorous). This might just be my stock (originally derived from Kath Dryden's plants).
I am down to only two adult tubers of the pure yellow and I have taken steps to cross - pollinate between the yellow plants in order to increase my stock but even the seedlings seem slower and more prone to damping off than the reds. If I succeed then I may eventually be able to supply plants.
I do have a rather nice pale lemon one which arose presumably from crossing with a white form, and various orange yellow ones as shown in Maggi's links, derived from crossing yellow with red. None of these are as pure a yellow as the wild type.