Use ColorSync Calculator when a designer, printer or project owner needs a numerical colour value converted between two known profiles. It is an advanced image or print workflow tool. An RGB triplet by itself is insufficient: the source profile, numeric representation and intended destination must be known.

If you are simply sharing a photo or trying to make two screens look alike, this calculator is usually not the first step. Keep the original file and confirm the receiving application's or print provider's required colour settings. Do not manually substitute channel values in an image as a general colour repair.

Prepare the conversion

Get the source value and its profile from the responsible document or project owner. Also obtain the required destination profile and rendering intent. A number sampled from an unknown screenshot is not a dependable substitute for an agreed source colour.

In ColorSync Utility, open Calculator. Configure the source colour space, intent and profile on the left, then enter the supplied components. For RGB, check the bit-depth and extended-range controls so the tool interprets those numbers in the intended range.

Set the destination space, intent and profile on the right. Read the output produced by that configuration. There is no single correct conversion of “128, 64, 32” independent of these choices.

Hand over a usable result

Supply the converted components with their numeric representation, both profile names and the selected intents. The receiving application must interpret the value with the intended destination profile. If two conversions disagree, compare those settings before changing the colour by eye.

A calculation supplies a value; it does not calibrate a monitor or guarantee a printed match. For colour-critical work, confirm the result through the project's agreed proofing process before approving delivery. Keep the original specification so a later change of destination can be handled from that source.

Source: Apple: converting colour values.