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PNG Colour Rendering on the Web

When is a colour not a colour? When it's in a PNG image file and you want to display it in a web browser!

I have recently been working on a website which includes a number of PNG images which are generated at run-time by a piece of VB.NET code. During testing we noticed a number of discrepancies between the colours being rendered in the image and the colours specified in the site's CSS file.

To make matters worse, the behaviour varied from browser to browser. Google Chrome behaved as desired - a perfect colour match. Internet Explorer appeared to render the colours in the PNG darker than we wanted. When we tried Firefox, the colours in the PNG were rendered with a different shade of the desired colour (a slightly purple tinge to our desired blue, in our case).