I'm not really that consistent - my Sims change clothes and hairstyles a lot, even if it means growing their hair from a pixie cut to mid-back length in one day
I do like to coordinate their makeup with clothes though, and some of my Sims have a 'favourite colour' they like to wear in their outfits but it's not a must. I do, however, like the idea of Sims having a certain style - punk/goth, girly, natural tones, serious/professional, comfy, etc. depending on their personality/story.
As for hair, the only thing I
usually pay attention to is when I give uniforms to employees, I usually try to find a hairstyle for the uniform that is realistic regarding their actual hair. For example, the long-haired employees often get some type of a ponytail, but if the employee has short hair, I just leave it at that
Oh, and also the hair colours. I don't generally change my Sims' hair colours, unless it's something obviously unnatural - but it isn't often, as I don't use CC, and the Maxis unnatural hairstyles are not really my cup of tea