If you go through your camera roll we are fairly sure your hair looks dramatically different in every one and that’s because of the light.
Most likely a lot of images you have are indoors with little thought to light and direction of the light so it’s hit and miss with your hair. Some houses put warm bulbs, some out warm, some 40 watt some 100 watt so the variation is huge. Now take that add a harsh light coming in the window directly on the back of your head and chances are your hair looks black. If the bulbs are cool then the image will possibly look flat. If you take a photo at midday the sun is directly above you and the top of your head will be lit but your face one big shadow.
The ideal situation in natural light is know as “the golden hour“ which is directly after sunrise ( which let’s be honest no one is getting out of bed to take a “casual snapshot” of your hair at 6am!) and again before sunset. The light is softer and slightly warmer and more diffused.
Get the sun behind the person taking the photo and you will get a warmer softer result.