Plants of Lewisia tweedyi as 'Ashford Yellow' were offered at the NZ Trillium Group weekend last year and I bought two. As I remember all those on offer were in bloom with consistently yellow flowers. Not a rich, golden type of yellow but definitely yellow, all the same as I recall. I had just 5 seeds from my two plants and all are up, small, unflowered plants as yet.
I thought I had lost the two originals as the foliage got a bit overgrown by other things and rotted but just last week I found both are growing away strongly from the centres.
I also bought a good yellow cotyledon form a couple of weeks ago at our OAGG Show. The colour is the same as that of the tweedyi plants.