I have the following logo, which displays as the same background colour as the HTML body in FF3, Chrome (#363636).
But in IE8 it displays a different, darker colour.
The top-rated answer here suggests a bizarre reset to the gamma value of 0.5181347 using pngcrush. This may work in some universes, but in ours your best path is to strip all color-space information from the PNG so that it is rendered purely in the same neutral RGB-triplet space that the browsers are using for colors in CSS:
pngcrush -rem cHRM -rem gAMA -rem iCCP -rem sRGB infile.png outfile.png
What this means for real color-heads is that the original color you designed may not appear the same on another monitor or operating system, but all your colors with the same RGB values will be rendered the same way for each user according to the browser+os they are on. Specifically, background or adjacent colors abutting a PNG will match, so you can design your site with interlocking images and colors.