working with jpegs

i've only been using the php image functions for about a week now. i understand that jpegs have their own custom color pallette. so whenever i want to add something to an existing jpeg, the only colors i have to work with are the ones that are already defined. hence the purpose of imagecolorclosest(). at least this is how i understand things. so, for example, if i'm working with a jpeg that mostly uses dark colors, attempting to add white text to it doesn't always come out as bright as i'd like. is there any way around this without switching to a different image format or making system calls to something more advance like imagemagick? is it not possible to expand the color pallette after doing an imagecreatefromjpeg()? -jr