It is a true variegation and may be passed through seeds is the flower stalks comes out from the variegation zone of the plant.
Some arco-íris present pure white instead of yellow or rose and pinkish and no yellow.
As time passe by we find more and more of these rare plants as plant lovers sow many seeds and give chance for those beauties to appear.
We saw a white for a arco-íris on a delicada plant in Taiwan. it came from seeds I sent to there.
They are very famous.
the below picture is the one I like the most.
a Dyckia monticola var. arco-íris and believe
they make an entire population (some 12 individuals) of arco-íris plants.
More can be found around and we are bound to see that.
Pictures will be shown here.