Christoph P.S: AFAIK GD can use libjpeg-turbo for jpg-processing, which has chrominance subsampling options.
That would then depend on what your server and PHP has installed, because you can't choose this from the PHP code. Besides, there are no options for this in PHP resizing functions ... Thats why, the only chance you would have to achieve something like this, would be to resize images with
exec() if your server has something like imagemagick built-in.
I would also recommend you see what I said about
image_resize_function in my previous post. In your case, you are mostly losing the contrast of the red because it is being resampled into otherwise dark colors and loses it's effect.