Many Meconopsis produce albinos and this is the white form of M. horridula (probably the form M. prattii). At least a proportion of any seedlings will be white and in the medium term one can select a plant that breeds true crystaline white.

PLEASE NOTE I HAVE GOT THIS ALL WRONG!

Friends who have traveled widely in the Himalayas and indeed even took me to China to see Meconopsis, say that Meconopsis horridula was broken up by Dr. Grey-Wilson into a number of separate species. These are all quite distinct species and grow in defined locations that are very large distances apart. This is therefore a white form of the species M. prattii. Meconopsis horridula itself has a very wide distribution from west Nepal right up through Tibet and  beyond its borders to the north (it is difficult in cultivation in the U.K.). There are a number of subspecies of M. horridula described, as well as the related M. prainiana, pratti. racemosa and zhongdianensis. For any Meconopsis enthusiast there is a most excellent account with comprehensive and brilliantly photographed images in Dr. Grey-Wilson's book THE GENUS MECONOPSIS from page 238. The book was published by Kew Publishing. Royal Botanic Gardens.
 I suspect many gardeners however will refer to these plants as 'Meconopsis horridula'  as do some major seed lists like those of the Scottish Rock Garden Club. All have really horrid spines which can make seed collecting unpleasant. In the wild there are all shades of blue, pale yellow forms, purple/mauve and pink forms as well as albinos and the anthers vary between bright yellow and white/grey. The spines (very variable) on the leaves may have a purple pigment spot at the base (as does M. rudis in the garden). This like M. prattii is widely established in the U.K. Both are easy probably anywhere and like all this group of plants are usually biennial and monocarpic.