Using Curves to make color adjustments
I keep noticing a lot of people mentioning adjusting photos. There is actually a very simple method for using Photoshop that gives you the points you need to make adjustments in curves. I got it from an old episode of Photoshop User TV (available as a free download on iTunes). Unfortunately I don't remember which episode. I strongly advise subscribing to the iTunes Photoshop User TV and Matt's Killer Photoshop Tips podcasts on iTunes.
I'm not a teacher, but I will list the steps I do in Photoshop CS3.
1. Open the photo you want to color correct in Photoshop.
2. Select the color sampler tool from the tool bar (this is under the eyedropper tool).
3. Click on the half white half black icon (create adjustment layer) on the bottom right of screen and choose threshold.
4. Click on the threshold graph and drag triangle all the way to the left (photo will go completely white). Then drag it back to the right until you start to see spots of black on your photo (this shows you the darkest points in your photo).
5. Click the OK button on the threshold graph. Then use your cursor to click on one of the black spots. This will leave a circle with a 1 next to it on your photo.
6. Double click on the icon in your adjustment layer (this brings the graph back up). Click on the triangle and drag it all the way to the right (photo will go completely black),. Then slowly drag the triangle back to the left until you see a big enough white spot on your photo that you can click on (this gives you the lightest points in your photo).
7. Click the OK button on the threshold graph to close it. Then use your cursor to click on one of the white spots. This will leave a circle with a 2 next to it on your photo.
8. Delete the threshold layer by dragging it to the trashcan.
9. Add a new layer (by clicking on the square with the corner turned up).
10. Go to Edit, Fill (or hold down the shift key and press the Backspace key). For content choose 50% gray.
11. Change the blend mode of the gray filled layer to Difference.
12. Click on add new adjustment layer (half black half white circle) and choose threshold (your threshold graph should start in the center and go to the left).
13. Click on the triangle and slide it all the way to the left (photo goes completely white). Then slowly come back to the right until you see spots of black. This shows you where the midpoint spots are in your photo (values between darkest and lightest points).
14. Click the OK button on the threshold graph to close it. Then use your cursor to click on one of the black spots. This will leave a circle with a 3 next to it on your photo.
15. Delete the two layers (threshold and the one filled with gray).
16. Click the add adjustment layer (zen circle) and choose Curves. Below the graph are three eyedroppers. Click on the eyedropper furthest to the left (this sets your shadows). Mouse over your photo and click on the circle with the 1 next to it.
17. Go back to your curves graph and click on the eyedropper furthest to the right (this sets your highlights value). Mouse over the photo and click on the circle with the 2 next to it.
18. Go back to your curves graph and click on the eyedropper beneath it that is in the middle (this sets your mid-tones value). Mouse over the photo and click on the circle with the 3 next to it.
19. Click the OK button on the curves graph to close it.
20. To remove the numbered circles click the clear button (under the menu names at the top of the screen).
This gives me perfect color almost every time, because it uses the color values in the photo.