Klara writes

white

best: G. phaeum 'Album' and G. sanguineum 'Album'
most unusual : G. pratense 'Striatum'
worth considering: G. clarkei 'Kashmir White'

pink

Most interesting foliage: G. x riversleaianum 'Mavis Simpson'
Assessors' favourite: G. x monacense var. monacense 'Muldoon'
Best for flower power: G. 'Dilys'
Best for front of a border: G. sanguineum 'Glenluce' and G. sanguineum
var. striatum
Best for early flowering: G. phaeum 'Lily Lovell'
Worth considering: G. 'Anne Folkard'

blue

Best space filler: G. 'Orion'
Best autumn display: G. wlassovianum
Best blue colour/ Long lived blue: G. 'Johnson's Blue' and G. 'Brookside
Worth considering: G. sylvaticum 'Amy Doncaster', G. pratense 'Plenum
Caeruleum'

no violet or purple, but I would have considered many of those they
call blue or pink more like purple. Try google images to see which you
like!

For propagation they only consider division, but you can pot up very
small ones...

Not on this list, but I can't remember the species - there's one which
has pink flowers with purple veins, flowers in abundance about 1cm
across, and another which is white with dark veins, flowers about 1.5
inches across - really nice for taking a close look at, whereas most
hardy geraniums are best for just providing a mass of flowers and
attractive foliage.

G endressei is excellent for sheer toughness (pink flowers)
--
Kay