[Tex/LaTex] Can \includegraphics be used to change an image color

Question

I'm including a number of PDF images in my document using \includegraphics. I'm wondering if there's a way for LaTeX to adjust their color profile, hopefully the same way it can flip images upside down, resize, and crop, as here:
http://en.wikibooks.org/wiki/LaTeX/Importing_Graphics#Including_graphics

Let me detail a little more my desired outcome, in case there's another way to achieve this:
The images are brightly colored—I'd like to reduce color levels, or possibly render them grayscale, and I'd prefer to not have to do external pre-processing, if this is possible.

I'm using XeTeX.

Answer 1

No, \includegraphics can't change the image color. You need to use an external image editor. The graphics/graphicx packages are more or less just interfaces which pass the image to the output driver, like dvips or pdftex. Therefore you are limited to the features provided by these drivers. Image manipulation is not part of these drivers and even if it would be very difficult to have an identical interface for these in graphics/graphicx.

For anything except resizing, trimming/clipping and rotating you need to use an external image editor. However, using PGF/TikZ you can place some overlays before and behind the image. If the image has transparent parts you could add a background color etc.