This tip is for editing photos more than illustration. But it can be used to adjust colors in an illustration as well.
Let's say you have a photo of a landscape, but the green of the grass is too bright and loud a shade of green, but the skyline is exactly as saturated as you want. What do you do?
The obvious solution is to lower the saturation of the entire image, this can be done with variations, a color overlay, adjust hue/saturation ..etc. But the problem is that if you adjust the grass to look more real by desaturating it - that lovely sky now looks too grey.
Now you think - AHA! I can mask off the sky and just apply my filters to the grass! But if you go ahead and do so - the image won't look right. There will be a hard line (or soft line) distinguishing saturation levels between grass and sky, doing your own mask will look fake no matter how carefully you render it by hand in an alpha channel.
So what do you do?
Use the channels menu! You have RGB - check each channel for one that naturally distinguishes the grass naturally. Change the mode to LAB color - to get another set of channels to consider.
When you find the channel that best isolates the grass without any tweaking - duplicate that channel. If the channel is a LAb channel - duplicate the chanel and then return to rgb space.
Use select - load channel - and select the channel you just duplicated. Apply your desaturation to this channel (invert selection if necessary). Because this selection is 'natural' - your desaturation effect will also seem 'natural' and no one will be able to detect the digital enhancement.
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