That's a well-known problem: extracting things from a pdf-file doesn't necessarily result in something that's useful for further processing. That's not only for graphics, that's for MS Word also. Think about converting a .pdf to Excel. You won't get the formulas!

Consider a pdf-file as to be useful for presentation (view on screen, and print if allowed by the author).
So the two things you can do:
1. Ask the author of the pdf for the original processable file.
2. Make a screen print, save that as jpg or bmp and use your favorite photo-editor (even MS Paint is useful to change colors) to edit it.

No need to be desperate. Photo-editors can do quite a lot.