Having a hard time trying to figure out exactly what color you need for a drawing or painting? If you are working from reference photos, there's an easy way I just discovered. All my reference photos, even ones I took myself, are digital. I use Linux primarily, not a Mac (though I do have one) and definitely not Windows. I also don't like to pay for software. Thus, I was ecstatic when my husband introduced me to Linux and open source software. Read: FREE (legally!). Instead of the pricey Photoshop I used to use (and still do occasionally), I now use the free GIMP which does almost everything that Photoshop does. GIMP is available for Mac and Windows, too.
Anyway, if you have software like GIMP or Photoshop that has a color picker (usually a little eyedropper icon), just use that to find out what color you are looking at. You can even see what proportions of red, green and blue make up that color. It was this way that I discovered that the pink I was looking at actually had quite a bit of green and blue in it. I layered aquamarine and blush pink and got just what I wanted. Until I learn how to pick these colors out with my naked eyes, I'll be using the color picker on my computer.
By the way, you can use this for black and white, too. Convert your color images to grayscale, then use the eyedropper tool to see what values you need.