I think she has the hair and skin type that wouldn't necessarily look good with grey or silver. She's got quite a fragile hair type and she's in a business where she can't let her hair grow out naturally... well she can but the pressures are immense and certain roles require certain looks.
I started getting grey hair very young, at 16 I had a small but visible grey streak... on the one hand it's really cool to have significantly grey hair before you're 40 on the other hand I can't just dye my hair any colour I like because regrowth is tricky and for the grey to look good you have to commit to it 100%.
It's the in-between phases where you're not grey enough that's harder to maintain. Naturally dark hair with silver highlights done properly looks good... with blonde or lighter hair the grey is so close in tone that silver highlights doesn't improve much and if you have fragile hair like Nicole dyeing it a shade closer to silver/grey is tricky.
Grey/silver hair also looks good if it's kept sleek... dry, frizzy, curly grey hair looks like a bird's nest.