Hi Katarina, Forgive me if I'm wrong, but from your question, I have a feeling that you're not clear on what an Alpha Channel is.
Computer graphics store their information in channels; you have one each for Red, Green, and Blue. You might have another for the Alpha, or others for selection masks, and so on.
When you look at the Layers palette, or at your image, you're seeing a composite of the Red, Green, and Blue channels. The Alpha channel doesn't show unless you want it to, in which case it shows as a mask.
So, yeah, what Chosen is talking about is the "invisible" alpha channel; but you don't make it with a lasso. You click on the Channels tab (usually grouped with the Layers tab in Photoshop,) click on the Make New Channel button (in PS,) which looks exactly like the Make New Layer button, and paint directly on that Channel.
It's as easy to use as a Layer, even though it's not one. All of your tools and brushes and many of the filters and third-party plug-ins work on it. You can increase or decrease the contrast, invert, equalize, posterize, or do all kinds of other stuff; but all in gray scale.
Once you have it, you can change the color of your hair at will, and it doesn't affect the Alpha channel in the slightest.
So I'm not sure what you mean about drawing the black brush over the hair. You're not painting on your hair layer at all; you're painting on the Alpha Channel. But if you're talking about adding black streaks to the hair color, once you have the Alpha Channel, the hair will look exactly the same in SL no matter what color you put there.
All of this assumes that you're not using PS 7.0 with its nasty embedded alpha. If you are, then the Alpha channel won't work properly. You'll need the free upgrade to 7.0.1, available on the Adobe site
here for the PC,
here for the Mac, or, if you need another localized language, you can find links on these pages, for
PC or
Mac.
Hope this helps!