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Originally Posted by bedlam123
mercury,
When working in Photoshop, there are many color modes to choose from. The more common modes that are implemented are RGB and CMYK. Undoubtedly, you've noticed that these color modes are a bit limiting. RGB will allow you to create amazing, vibrant blues, dark greens and purples. Great on-screen, but when you try to print these colors to a CMYK printer, they look as if the black ink cartridge threw up all over them. Also it's nearly impossible to create rich yellows and oranges using RGB. CMYK is limited in color representation. If you have to print, do so in CMYK. This is usually the determining factor when deciding on a color mode to work in. But, you do have another option that has an endless color selection--Photoshop's Lab mode.
In Photoshop, the Lab mode consists of three color channels. The first channel is Lightness (L). The Lightness component, also called luminance, can range from 0 to 100. A Lightness value of 0 equals black and a value of 100 equals white. The higher the value, the more vivid the color. The other two channels, a and b, represent color ranges. The a channel contains colors ranging from green to red and the b channel contains colors ranging from blue to yellow.
When working in Lab mode you're provided with a wide range of color values to choose from. The Lab color model actually includes all of the colors that you can create in both RGB and CMYK modes.
Working in Lab mode is just as fast as working in RGB mode Photoshop has only three channels to work with when processing information, but working with CMYK images can be much slower. Remember even though you're working in CMYK mode, your monitor is RGB so Photoshop is converting these values continuously.
Because Lab mode is device-independent, you can use it to edit any image and your colors won't undergo major tonal changes when converted to CMYK for final output. This is one reason many of the high-end users prefer to work in Lab mode.
There really are no limitations when selecting or editing colors on-screen if you're working in Lab mode. Although, if any of your colors fall out of the CMYK range, you may have to fine-tune them a bit. This tends to occur when you're working with very vibrant shades of blue and purple. A way to avoid any color alterations is to print to a PostScript Level 2 or Level 3 printer. You can print an image in Lab mode directly to either one of these printers to avoid any color conversions.
Try this, open a color image in Photoshop and convert it to Lab mode. To do so, choose Image > Mode > Lab Color. Then click on the Channels palette (if the palette is hidden, choose Window > Show Channels), and select the a channel. This channel maps the greens and magentas. Invert the colors by choosing Image > Adjust > Invert, or by pressing cI ([Ctrl]I in Windows). After inverting the a channel, click on the top-most channel, Lab, to see the results. Repeat these steps for the b channel.
Not only can you invert colors using channels, but you can even replace a color. Open a new image in Photoshop and convert it to Lab mode. Select the a or b channel and then choose Image > Adjust > Replace Color. In the Replace Color dialog box, select a color that you want to replace by clicking on the preview window with the Eyedropper tool, or by clicking on an area of color directly on your image. Set the Fuzziness slider to 40 and choose the Selection option button. Then drag the Lightness slider back and forth to achieve your desired result. Click OK to apply your changes. Again, select the Lab channel in the Channels palette to see the final results.
This explanation was rather lengthy, but I think you can see now that using Lab will offer you some extra versatility.
There are a large number of web sites where you can get even more info if this was not enough.
Happy New Year!
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