Making green really green (Photoshop)

One photoshop tip that I tend to forget from time to time is from Dan Margulis, and have appeared on the colortheory mailing list. Here it is, copied verbatim from Dan’s mail and also accesible via the mail list archive. BTW, do buy and read Dan’s books if you’re into colour theory. They are really excellent, and will keep you busy for days…

Folks,

Since finishing PP5E, I’ve been putting together some of my own photo albums so that my wife and I can remember some of the interesting places we’ve been over the years. I’ve worked on several hundred images, mostly mine, some taken by others.

This has been an interesting learning experience, because in recent years when I was correcting files it was usually because somebody else had not been successful with them or because in looking at the image I wanted to try out a particular Photoshop technique. This time, the images are more “typical”. I’m working on them because I want to make them look better, not because they pose any particular technical challenge. Also, I’m not shooting for perfection–I’m trying to limit each correction to five minutes, although in practice they often take much longer because I correct them several different ways to test various techniques.

Anyhow, I’ve now found some better ways of doing certain things than described in PP5E. There are three such areas so far. I’ll share them with the list but am not yet ready to discuss two of them because I’m still testing them against other alternatives to see exactly how much better they really are. However, I’m ready with the following, which affects almost anyone who shoots nature and is looking for vivid greens.

If the original has pretty good greens to start, particularly if they are differentiated greens (plants of different species) then the best option is still LAB to drive them apart while intensifying them. Problems can develop when the original greens are too brownish, though.

At various times on this list, and in pp. 231-234 of PP5E, I’ve discussed using Channel Mixer to intensify greens, an idea first suggested to me off-list by Jerry P’Simer. My previous view was that the method could create problems in non-greens and that it should be used primarily as a desperation measure when areas that should be green don’t measure as green at all, such as the tree in PP5E’s Figure 10.10A. It would have been troublesome to take that into LAB because the tree was so neutral that it didn’t measure A-negative. I showed how to use Channel Mixer to make it just green enough that we could now move the file into LAB and make a pretty good picture out of it.

It turns out that there are a couple of safeguards that transform this method into something that can be used much more frequently. I now tend to use it as a start in any image where I’d like to put more pop into greens, not just where the greens start out nearly hopeless. I still will use LAB to differentiate greens where necessary afterward. Here goes.

Problem: We often remember greenery as being considerably more saturated than the camera saw. Clients also clearly prefer greener, rather than duller, lawns, trees, and other plant life, regardless of what is found in the original capture.

If the affected area already starts out somewhat green, then we have LAB, Hue/Saturation, and/or Selective Color, among other things, to intensify it. But some “greens” are so desaturated on capture that Photoshop doesn’t recognize them as greens at all, but rather dark yellows.

Solution: for maximum flexibility, here’s the workflow:

1) Starting with an RGB file with poor greens but no obvious color cast, on a duplicate layer (not an adjustment layer) Image: Adjustments>Channel Mixer. Choose Greens and enter Green +140%, Blue -40%.

Explanation: The procedure of subtracting part of a channel from another is usually not advisable because it costs detail in darker areas (the darkest areas of the channel are subtracted more heavily than the lighter parts). In natural greenery, however, the blue channel is usually almost solid, so subtracting doesn’t harm detail in the target channel.

The procedure does not harm gray balance, because in a gray, all three RGB channels have equal values, so adding 40% of the green while subtracting 40% of the blue results in the same color. In a naturally green or dark yellow area, though, the green channel starts out much lighter than the blue, so adding 40% of green while subtracting 40% of blue gives a lighter (greener) result in the green channel.

40% is chosen because I have not seen any examples where a higher value was appropriate, and we can always reduce it later. An adjustment layer should not be used because it rules out the possibility of taking the layered document into LAB.

2) After applying the above Channel Mixer command, choose Edit>Fade: Lighten.

Explanation: Without this step or something that does the same thing, blues such as skies tend to go purple because the green channel is getting darker. The Lighten command restricts the effect to areas where the green channel is getting lighter, as they will be in any areas that are greenish in nature.

The Fade command is used instead of changing the layer mode because that command is needed in step 4. You can retain the flexibility of letting the skies go partially purple by adding another layer to the process. However, I have yet to encounter an image where I wanted any part of this effect, so I don’t mind eliminating it altogether with the Fade command.

3) Carefully examine the top vs. the bottom layer to see whether there is an undesirable change in any color other than green. The most likely culprit is that certain browns may get too yellow. If you see this happening and consider it a problem, make a mental note of it but move on.

4) Change the top layer’s mode to Color and decide whether the image now looks better. If not, cancel the command.

EXPLANATION: The Channel Mixer move makes green areas greener but it can also make them too light. Color mode restores the original darkness while retaining the added greenness. Sometimes this looks better and sometimes not, in which case you return to Normal mode. I’d guess I use Color mode for about 2/3 of my images and Normal mode for the other 13.

5) Because the settings of 140% and -40% are excessive for most images, reduce the opacity of the top layer to taste. If it’s a big reduction in opacity, you may wish to re-think your Color vs. Normal decision of Step 4.

6) If you decided in Step 3 that colors other than greens are being damaged by the move, use Convert to Profile to convert the document to LAB, without checking the Flatten Image box–the two layers must remain separate. Now use layer Blend If options to eliminate the offensive colors.

EXPLANATION: The colors that are being hurt are presumably browns and reds. Such colors are always A-positive, whereas greens are A-negative. You can therefore restore the bottom layer in those colors by bringing the Blend If slider in to exclude anything that’s A-positive (more magenta than green).

Dan Margulis