That's wonderful! I have a few denisoniana in my collection ... almost all of them have flowers that are pale yellow when they first open but within a week, the color intensifies to mustard yellow or orange-buttercup.
Is your denisoniana a clone?
It is not a clone. In fact, I have 3 V. denisoniana of this color that I bought in a local market but on 2 plants there are few spots on the flowers as it occurs often with this specie. This plant is my favorite as no spot can be seen.