Why do values not match those in my image editor?

Classic Color Meter, like all system-level color meters, only sees the values that macOS sends to the display. These values are in the display's color space. As an image editor has access to color information prior to any color space conversion, its meter may choose to display this information as raw values in the image's color space.

For website and application development, it's recommended that you set both Classic Color Meter and your image editor to the sRGB (or Display P3) color space.

For more information, read my article on macOS Color Meters and Color Space Conversion.

I'm a photographer. How accurate is Classic Color Meter's L*a*b* display?

As mentioned in my article, there are two types of color conversion issues: clipping errors and rounding errors.

Clipping errors occur when your computer's display uses a narrower color space than your camera. For example: your camera is capable of Adobe RGB but your monitor is sRGB or Display P3. Using an Adobe RGB monitor will eliminate (or drastically reduce) these errors.

Rounding errors occur due to limitations in macOS. Specifically, macOS may reduce the color depth to 8-bit even when your photo software and display support higher bit depths.

That said, as long as your display uses an equal or wider color space than your camera, the L*a*b* readout should be reasonably accurate and usable.

What are the “legacy” features that are hidden by default?

Starting in version 2.0, Classic Color Meter hides certain legacy color spaces and conversions by default. These were used by a small percentage of workflows and mostly fixed quirks in macOS's color management.

To re-enable these options, select “Show legacy color spaces and conversions” from the Advanced pane in Preferences.

• RGB, 16-bit

Historically, Apple's Digital Color Meter app offered a "16-bit" option which displayed each color component as a number between 0 and 65535.

Classic Color Meter added this feature for parity. Unfortunately, users thought that the "16-bit" mode was more accurate than the "8-bit" mode. In reality, Apple's screenshot API has always returned 8-bit values (even on 10-bit displays).

• Show as Generic RGB

This option converts colors into the non-standard "Generic RGB Profile". As far as I can tell, this color profile was last used in the Mac OS X 10.3 Panther era (released in 2003).

• Convert to main display

Around Mac OS X 10.7, Apple would convert colors on the secondary display to the main display's color space. This option was occasionally useful in that era.

Why do colors in the right view appear less saturated than those on the left?

Newer Macs feature a display with a P3 color space. This provides deeper colors than the sRGB color space.

When Classic Color Meter is set to “Display in sRGB”, the closest sRGB color will be shown on the right. The original screen image is shown on the left. This sRGB color represents what most Macs and iOS devices (which have sRGB displays) would see.