I am drawing a picture of myself.......I have really dark brown hair that almost looks black if your not close enough. sHOULD i paint it all brown, or should I mix in black with brown to make the dark brown more visible....help.......

I would say try to make it as realistic as possible. If that means mixing colors, do it.

Pure black in a painting will create a very flat unrealistic effect - if this is what you want then OK, but if you are trying to be realistic than i recommend you mix your own dark colours and mid tones, I never let my beginner art students use black or brown straight out of the tube- i want them to learn about mixing colours and so teach them how to make their own black and brown - these self mixed colours have more depth and versatility -
to mix a "black" in oils or acrylics use Alizarian crimson+ultramarine blue+viridan green - or use a combination of your darkest red, darkest blue and a little yellow

for a brown - mix 3 primary colours together- i.e Cadmium Red+Cadmium Yellow+Cobalt Blue

To lighten the brown add more yellow, to darken add more blue and red

Avoid overusing white for the mid tones and highlights this will only make you look like you have grey hair

Instead of just brown and black I would mix about 5 shades of colour ranging from dark to light, keep looking in mirror, very dark hair can reflect a lot of colours, you may see a hint of blue in the highlights.