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Colour blending conundrum

Update (19th March 2016)

I came across this argument debatePhotoshop's gradient editor needs a few improvements—that goes into more detail on this issue than anyone is likely to care for!

Update (21st November 2022)

Björn Ottosson’s article How software gets color wrong demonstrates the problem and provides some explanation for why it occurs. The article does not, however, offer any enlightenment as to what approach Microsoft chose in order to prevent it.

Through developing and working with Colour Selector, I came across an interesting problem with the way it blends various colours, including red (#F00) and lime green (#0F0), and yellow (#FF0) and blue (#00F). You can see from the following screenshot that a 50/50 blend of red and lime green comes out as far too dark a shade:

Colour Selector reports that the value (the brightness of the colour) drops from 255 (100%) of red and lime green to 126 (49%) for a 50% blend of red and green. That cannot be right. I’ve ruled out errors and naivety in my code, because I get the same results from Eric Meyer’s Web-based Color Blender tool:

Photoshop 5 and Mac OS 9 both make the same mistake (as does Mac OS X Tiger):

I then tried Windows Paint as a last resort, which was the first test that succeeded:

GDI clearly knows something that the rest of us don’t know.

I can’t test this in all the different GUIs (and text rendering libraries) myself, but to satisfy your curiosity, here is a test running on your own system right now:

This is a test

I never have managed to find out what this is all about.