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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!

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.