Use the secondary color correction tool JetDV has a good tutorial on this on his site, there is a similar one but not as specific to your question on the VASST site.
There have been many posts about affecting just one color. If you don't find everything you need at the excellent site referred to above, you can click on these links:
Follow the advice, except use the color wheel to change the selected color instead of changing it to black and white. If the result isn't perfect, you can sometimes use a mask to reduce the area over which the Secondary Color Corrector must operate.