I have printed embedded coloured pictures from Gmail happily in colour, but now it's changed so that while I can still see them on the screen in colour, when I print them they preview and print in greyscale.
It's likely that your print settings are set to greyscale. Now, I can't provide specific advice, because each printer driver / manufacturer presents the options in slightly different ways, but it's common for a color printer to offer the option to convert to greyscale (in order to save on CMY ink, for instance). Try exploring the print dialog's dark, deep corners to see if some greyscale option accidentally or spontaneously got turned on.
Of course, this could be a completely unrelated issue. One clue is to see if there are any other color elements on the page which are also turning grey, and to test printing from other websites.