from Black to brown, or dark brown to lighter brown

I usually use selective color and mask out what I need changed.

can you show an example for me?

Here is a tutorial for turning dark hair blonde. Same technique should yield light brown.

The biggest issue I see with that image is that it's eyebrows, so you need more careful masking than you would for hair. You're not just changing color, you're lightening a good deal, and you don't want to lighten the skin, or hello? Photoshop.

You may have to do some channel-based masking in order to isolate the adjustment to the eyebrow hairs alone.

What about Blonde to Dk Brown on a black and white image?

For the mask: Calculations - Red,Green in Overlay mode (100%); invert the channel; then Apply Image Red channel (inverted) in Overlay(? can't remember now). Feathered the bottom of the mask with the smudge tool to keep the shadow side shadowed.

The most shadow-lightenening/midtone protecting tool is of course Image>Adjustments>Shadows/Highlights, so I ran that on a duplicate layer, masked with the alpha channel just created. A Solid Color fill layer above that for hue, then a Soft Light layer above that with some white dodging.

Sorry to be so vague about the process, I took a long phone call right as I was finishing up, and can't remember exactly what I did.