In order to correctly output images on different occasions, it is sometimes necessary to convert images from one mode to another. Photoshop converts the desired color mode by executing commands in the "Image/Mode" submenu. This color mode conversion sometimes permanently changes the color values ​​in the image. For example, when converting an RGB mode image to a CMYK mode image, the RGB color values ​​outside the CMYK color gamut are adjusted out of the CMYK color gamut, thereby reducing the color range.

Since some colors will lose part of the color information after conversion, it is best to save a backup file for them before conversion so that the image can be restored if necessary.

1. Convert a color image to grayscale mode

When you convert a color image to grayscale mode, Photoshop throws away all the color information in the original image, leaving only the pixel's grayscale level.
The grayscale mode can be used as an intermediary mode for converting between bitmap mode and color mode.

2. Convert other mode images to bitmap mode

Converting the image to bitmap mode reduces the image color to two, which greatly simplifies the color information in the image and reduces the file size. To convert an image to bitmap mode, you must first convert it to grayscale mode. This removes the hue and saturation information of the pixel, leaving only the luminance value. However, because there are only a few editing options available for bitmap mode images, it is best to edit the image in grayscale mode and then convert it.

The bitmap mode image edited in grayscale mode may look different when it is converted back to bitmap mode. For example, a pixel that is black in bitmap mode may be grayed out after being edited in grayscale mode. If the pixel is bright enough, it will turn white when converted back to bitmap mode.

3. Convert other modes to index mode

When you convert a color image to an indexed color, many colors in the image are deleted, leaving only 256 of them, the standard number of colors supported by many multimedia animation applications and web pages. Only grayscale and RGB mode images can be converted to indexed color mode. Since the grayscale mode itself is composed of 256 gray scales, there is no significant difference in color or image size after conversion to indexed colors. However, after converting an RGB mode image to an index color mode, the size of the image will be significantly reduced, and the visual quality of the image will also be somewhat impaired.

4. Convert RGB mode images to CMYK mode

If you convert an image in RGB mode to CMYK mode, color separation occurs in the image, and the color gamut is limited. Therefore, if the image is in RGB mode, it is best to edit in RGB mode and then convert it to CMYK image.

5. Use Lab mode for mode conversion

In the color modes that Photoshop can use, Lab mode has the widest color gamut. It includes all colors in the RGB and CMYK color spaces. Therefore, using the Lab mode does not cause any loss of color. Photoshop uses Lab mode as the internal conversion mode to complete the conversion between different color modes. For example, when converting an image in RGB mode to CMYK mode, the computer first converts the RGB mode to Lab mode, and then converts the Lab mode image to the CMYK mode image.

6. Convert other modes to multi-channel mode

Multi-channel mode can be obtained by converting the color mode and deleting the color channel of the original image.

Convert CMYK image to multi-channel mode to create cyan, magenta, yellow and black spot colors (spot color is a special premixed ink, used to replace or supplement printing four-color ink; spot color channel is available for image preview preview Image of a dedicated color channel).

Converting RGB images to multi-channel mode creates images composed of cyan, magenta, and yellow spot colors.

Deleting a channel from an RGB, CMYK, or Lab image automatically converts the image to multi-channel mode. The original channel was converted to a spot color channel.

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