Why does overprint preview affect spot colours without overprint?

[flash_test_dummy]

Hello

I am working on a plugin that needs to calculate the colour of RGB spot colours with different tints. With no overprint preview, the colour is straightforward to calculate by appling the tint value taking the spot colour as 100% tint and white as no tint, e.g. if the red value was 175 and there was a 75% tint, the final red value would be 175+(255-175)*(1-0.75) = 195.

The problem I am having is that when you turn on overprint preview the colours change even when the objects do not use overprint. I have created an example file that has a Grey 195 process colour and three spots with tints that produce the same colour when overprint preview is turned off. When you turn on overprint preview the spot colour shades change, but the process colour does not.

Without overprint preview

With Overprint Preview

If anyone can explain what is going on and ideally how I could use the SDK to calculate the colours with overprint preview turned on that would be very helpful.

Thank you

[flash_test_dummy]

Doing some more searching it seems that as well as enabling overprint for objects that overprint, turning on overprint preview also does some colour space conversions, so I guess my question is really:

How can colours that use the same colour space (RGB) and are the same colour without overprint preview change to different colours when overprint preview is turned on?

[flash_test_dummy]

Seems the colours change because it does the colour space conversions on the full tint value and then applies the tint, rather than the other way round. Anyone know what colour space conversions apply to spot colours when overprint preview is turned on? Looks like it might be going through LAB at some stage.