Actually black hair all round is the Sisters archetype, or at least uniform hair colour. It's part of their whole 'Nuns with guns' ethos. Individuality and concern with personal appearance is not important. Instead presenting an impressive front to cow the enemy and show your devotion to the Emperor is what is important. My large Sisters army has black hair all round. I just presume they dye it.
The reason they look like Blondie is because of the pale face and bob haircut. This is part of the whole Nun thing. Nuns cover themselves up, so are pale-skinned, and they aren't bothered about their hair. Bear in mind that real medieval Nuns shaved their heads under that wimple (the Nun headgear).
If I wanted a genuinely 'feminine' army instead of a bunch of ascetic fanatics (which is what the Sororitas are) then I would collect a Guard army based off of the Escher gangers from Necromunda. These are warlike women who are nevertheless still consciously women. The Sisters represent an ideal of devotion, a female one. We in our world so habitually tie up women with their sexuality that we find it hard to comprehend that there may be people who do not see this as important. The Sisters are female, but not sexual.
They also don't look youthful to me. Their dour and fierce expressions look more like those of hardened veterans. I think of my Sisters as being mainly in their thirties and forties age-wise (Imperial juvenat processes likely mean that some may be far older). The smooth skin of their faces is due to a good diet, no tanning and regular exercise. Women in our society chronically ignore such things and so age prematurely.