1) Open your color photo in Photoshop and adjust the levels
so that the image appears as you would want it, if you were
to keep it as a color image.
2) If the photo is not already in 8 Bit RGB mode, convert
it to 8 bit RGB mode now.
3) Under the Layers Palette, click the Adjustment Layers
icon (the half black circle at the bottom of the palette).
Click on Channel Mixer from the menu to make a Channel Mixer
On a side note, you should use Adjustment Layers for your
edits whenever possible because they allow you to modify your
image in a non-destructive manner. This means that when you
are using Adjustment Layers to make your image brighter, less
blue, B&W, or whichever edit you are choosing, the underlying
image is never altered. Photoshop just keeps track of the
edits you are making on the adjustment layer and applies them
to the images underneath for display on your monitor or for
exporting. The advantage here is that since the data in your
underlying image is never altered, you can make as many edits
as you want without degrading the image, remove your edits
altogether if you change your mind later on, or in the case
of this exercise, selectively apply them.
4) Click the Monochrome checkbox on the Channel
Mixer panel. This tells the Channel Mixer to convert each
pixel to a grayscale value based on the color information.
And because the color information is still there, you can
use this information to adjust the way the image will look
by weighing the colors differently, which is basically what
happens when you use color filters in the analog world. Just
slide the RGB sliders to choose which colors you want to use
to determine the grayscale values. For instance, if you set
you Red channel to 100% and the Green & Blue channels
to 0%, this is similar to taking a B&W photo with a deep
red filter because only red light is being used to render
the B&W image.
The RGB ratio of 30/60/10 usually works well
for most pictures. If you want darker skies though, a little
more red and less blue should be used. In some instances such
as the in the photo of the Cham
Towers of Po Nagar in Vietnam, I will gain the red channel
to over 100% and the blue channel into negative range to get
a pseudo-infrared effect. For portraits, too much red will
make people look like they're glowing so emphasis on the green
channel is usually best here. Play around with the ratios
to see which one you like best but keep in mind that you'll
usually (but not always) want the ratios to add up to ~100%.
5) Take a look at your layers palette.
You'll notice the Adjustment Layer comes with a Layer Mask.
This mask is represented by the rectangle on the right side
in the Adjustment Layer. The Layer Mask allows you to select
the areas of the image your Adjustment Layer affects. This selecting
is done by painting the Layer Mask. The areas on the Layer Mask
which are painted black will block out the effects of the Adjustment
Layer while the areas painted white allow the Adjustment Layer
to affect the images underneath. The effects are not binary
so you can use shades of gray to paint areas which you want
to be effected by the Adjustment Layer to a lesser degree. By
default, the entire Layer Mask is painted white so the Adjustment
Layer affects the entire image.
When painting the Layer Mask, make
sure it is active. You can determine this by looking in the
box to the right of the eye icon. The square symbol with a hole
in it indicates that the Layer Mask is active. If you don't
see the symbol, click on the Layer Mask icon (the rectangle
to the right of the Adjustment Layer icon).
6) With the Layer Mask active, paint
the areas you want returned to color with black paint. When
your layer mask is active, the paint doesn't go on the image
but the Layer Mask so don't worry about it putting black on
your image. As mentioned above, the black paint is used to block
out the effect of your adjustment layer, which in this case
is the Channel Mixer trying to make your image B&W. Set
the hardness of your brush below 50% so that the transition
edges don't look too hard. If you make a mistake painting the
layer mask, remember you can always paint that area with the
opposite color to undo the mistake. To see how the Layer Mask
looks, click the Layer Mask icon while pressing Alt.
Your photo is now only grayscaled
in the areas you want it to be.