Color correction based on known samples (that are not grey)

Question

In Lightroom and other such programs, color temperature for lighting is easy with an eyedropper tool if you have a true neutral in the photo, such as a grey card. For a natural object, it gets you close enough to fiddle with, usually.

What about matching a known color that's not grey? For example, an article of clothing that can also be shot as an exemplar at another time.

There are color plaques used for photography, some quite pricy. How are they used other than by manual fiddling by eye?

Furthermore, the color temperature + tint model doesn't seem to be as effective with various indoor lighting sources, which I suppose don't have nice black-body spectra.

Is there a tool (preferably to use with Photoshop) that can do such color calebration?

---

Clarification: I know how to use the gray card (I prefer a cloth to get different angles at one go) in a preliminary shot (implied by mentioning the eyedrop-on-gray tool). My question is specificly how to handle not having that, and bad large-area lighting as well.

Answer 1

You can do this manually if you know what the color is supposed to be. It is similar to the process of calibrating to a neutral sample. The results will not be as accurate as if you use a grayscale, especially with mixed color lighting.

You should work from a RAW file if available.

1. First, set a best guess white balance.
2. Measure the known color in your image in terms of R,G, and B (Red, Green, Blue) values
3. Compare the measurement to your target
4. Adjust the white balance accordingly
5. Repeat from step 2 until the color is matched

Example: if the sample is 55,87,123 and it is supposed to be 55,82,125 you will leave the red alone, take out some green, and add a little blue.

Hint (added after your followup) to make RGB edits with temp/tint:

• To add red: Add yellow and magenta.
• To remove red, remove yellow and magenta.
• To change blue, use temp.
• To change green use tint.

Caution: when sampling the color, especially a fabric use a large sample (example 20x20 pixels) instead of a point sample to get an average reading.

Be aware that there are color shifts that can occur at certain hues and values, for various reasons, that will affect some parts of the image but not others.

Another tip: If you want to save money, get some paint sample cards from a home improvement store, including neutrals, and paste them to a piece of foam board. This would be better than calibrating to a shirt.

Answer 2

Ouch... You have a nightmare scenario.

I'm making some initial questions just as an excercise of posible scenarios.

1) You know the color of the samples... but Are the ilumination angles the same?

2) Is the response curve diferent between shoots?

3) Do a linear editing of the curves enought? or should I modify the gamma of each channel? How about the levels and not the curves?

4) Do I need to match just a plain sample or that sample in good light, and the shadow area aswell.

This basicly will be try and error aproach.

A basic methodology

• Copy your initial layer. Leave the background as a reference.

• Make some initial adjustments on this second layer. Use correct names for them like "More Red 15%"

• You can "over-do" a little the correction. Later if you play with the opacity of the corrections you can match it better.

This oldie animated gif was intended for other tests but the idea is the same. The first image has an exagerated efect and the second one has no efect at all. Play with transparency and choose.

• Working in layers will help you adjusting large bad iluminated areas, aplying masks. You make an exagerated correction and mask with gradients for example the amount of correction aplied.

• You also can make some layers with notes, for example a square indicating your area of interest, so you keep measuring that part of the image.

Answer 3

Is there a tool (preferably to use with Photoshop) that can do such color calibration?

Try looking at iCorrect EditLab Pro http://www.pictocolor.com/editlabpro.htm

It has a feature to do exactly that. Here is how to add a "Memory Color" - I'm picking one of the colours from ColorChecker reference rendition to sRGB, the "purple", and after Opt/Alt-Click on a free slot (I used the one to the right from "S" - skin, it is highlighted) a dialogue appears. Press "Set from Hue Handle", fill the Name and Label fields, optionally save the preset, and press OK. Now some other file may be opened and the saved hue edit preset applied. It is better to keep colour space consistent, that is if the custom colour is memorized from an image in sRGB, to apply it also to sRGB images.

Answer 4

If you have a image that you think requires colour balancing and has no obvious reference point to select, ie no white, black or grey, the you can temporarily covert the image to black and white and look or or use the White balance picker to select a netral grey area. Remember that spot and recovery to colour and select it with the picker. Grass, road surfaces, suntanned Caucasian skin all are mid tones.