If you shoot in RAW format you generally shouldn't need to use this technique as a relatively accurate colour reproduction can be acheived using the white balance tool. You may have also calibrated the white balance function within your camera. However, if you have old slide or negative scans in JPEG or TIF format accurate colour castes can be difficult to produce with much of the process involving guess-work. This is a semi-automated technique which allows Photoshop to do the guess-work for you.

The image I've chosen in the example below is a scan of a slide I took a few years ago on the shores of Loch Fyne in Scotland. The scanner used wasn't the best quality and so the photo has a distinct blue colour caste. This can be corrected quite effectively with this technique.

1. Create a copy of the background layer (Ctrl+J):

2. Apply the filter Blur > Average to the layer copy:

3. Select the background layer again and create a Threshold Adjustment layer:

4. Turn the visibility of the layer copy off. The image will now show the black and white regions of the image. Select the new adjustment layer so you can see the histogram:

5. Drag the histogram slider all the way to left then slowly move it to the right until you begin to see areas of black appear - these are the darkest areas of your image. Zoom in so you can see individual pixels:

6. Select the Eyedropper tool, hold down the Shift key then select one of the black pixels - this is simply to mark the black area:

7. Repeat the process in the white areas by dragging the histogram slider all the way to the right and then a little to the left to reveal the white pixels - again, zoom in, select the eyedropper and Shift-click on a white pixel:

8. Delete the Threshold Adjustment layer - you should now have 2 layers, the Background and the blurred copy:

9. Select the layer copy and apply a Levels Adjustment layer:

10. Select the new adjustment layer then zoom in to the black point you selected with the eyedropper tool - it should be labelled with the number 1). Select the black point eyedropper on the levels histogram and click precisely on the black point:

11. Do the same with the white point eyedropper remembering to zoom in so you can click precisely on the correct white pixel (labelled number 2):

12. Turn the visibility of the layer copy on:

13. Making sure the threshold adjustment layer is selected, select the gray point eyedropper and click anywhere on the layer - the colour should change to a neutral gray:

14. Turn the visibility of the layer copy off to reveal your new colour corrected image. If the correction appears too saturated or bright in some colours you can turn down the opacity of the levels adjustment layer a little.