Remove Color Casts Using Photoshop Lab Color

Color Correction is one of those things you don’t get a lot of practice on as a web designer, so I thought we might look the quick and mostly foolproof method I prefer to use.

Let’s start with a question. What is this? (see blue chip to the right). If you said, "it’s a particularly pleasant shade of blue," you’d be at least partially right.

However, you’d be only be 100% correct if you said, "a zoomed up section of Cameron Adams’ cheek," for, as you can see the photo below, the man in blue was the man of blue the day this particular picture was taken.

Now, this is a great photo, but it presented problems when I came to use it as his author shot on a book cover. While the blue tint looks pretty cool in isolation, it comes off a bit odd when placed alongside a handful of normal, pinky-yellow toned author shots. I needed to moderate the blue tint (or more correctly "cast"), but as the chip above shows, there was a lot of it!

Last year, you might remember we looked at using Photoshop’s Lab color mode to give any photo a visual shot in the arm.

Today we’ll look at LAB’s ability to make quite dramatic but highly targeted corrections to problem colors without destroying the overall color balance.

1). First, we need to change the photo from its default RGB to the LAB color space (Image/Mode/Lab Color). Feel free to save the image above and follow along in Photoshop.

2). As I explained in my first piece on Lab color, in the Lab universe, everything is explained in terms of three channels:

• L – Lightness vs Darkness
• A – Redness vs Greenness
• B – Blueness vs Yellowness (I remember ‘B’ for blue)

If any of those colors start to dominate an image, the quickest way to restore the color balance is to boost its opposite color.

As you can see by the screenshot at the right, the B channel is showing almost all its tone bunched up on the blue (left-hand) side of the chart. Rebalancing this channel will go a long way towards fixing this image.

3). With your image selected, bring up the Curves dialog (Image/Adjustments/Curves) and switch to the B channel.

4). Next, grab the top right (or yellow) corner of the graph and drag it slowly to the left, watching the blue cast start to rinse away as you do.

As you can see in the example at left, while the skintones begin to clear immediately, the true blues and reds are only marginally affected by the change.

5). Okay, the skintone is better, but moderating the blue has brought out a slight greeny tinge. We can tackle that by switching to the A channel and boosting the red just a tiny bit.

You’ll need to be a little sensitive with this channel as the blue wall behind Cameron has quite a bit of red in it, so adding too much red will start to turn the background towards purple.

6). Lastly, we’ll make a final adjustment to clean up the L channel.

Take each end of the line and slide it slowly horizontally back towards the centerline of the graph.

There’s no mathematically correct formula for placing your points, but:

• Your shadows should look richer without getting too "blocky," or filling in.
• Your highlights should look brighter without showing a visible edge or step between the lightest tone and the second-lightest tone.

When you’re happy with the result, click OK and feel free to switch your color mode back to your normal color mode (presumably RGB or CMYK).

I’ll leave you with an overlay of the original image on our finished product. Not a bad result for a two-step process (i.e. change color mode, adjust curves).

Published in Design View #42

Related posts:

1. Quick ‘n’ Dirty Color Correction with LAB Color
2. Color Blend Mode in Photoshop
3. Two Ways to Remove a Color Cast from a Photo with Photoshop
4. Feel the Color Burn
5. Multiply Mode in Photoshop

This entry was posted on Wednesday, February 20th, 2008 at 11:07 am, contains 670 words, 13 images, and is filed under Web Design. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed. The views and opinions in this blog post are those of its author.