It depends on the program you're using, but in Photoshop you can use the fill tool and set it to non-contiguous fill (i.e. it will change every instance of that colour in the image). Make sure the tolerance is set to 0 so that it only affects the exact colour you want to change and then just click on a pixel in the outline.
If the colour appears anywhere else in the image then you'd also have to select the area you want to change with the marquee tool first (in this instance you could draw a box round the head) so that it only affects that part of it.
My guess is that most modern software will have an analogous approach.
(I'm still using CS5 so the exact layout won't be the same anymore, but Adobe programs are pretty intuitive)
