In photoshop 7, how can I change the color of text that has already been saved as an image

Open up the image in Photoshop and select the Magic Wand tool, or press 'w' for a short cut. You can then use this tool to select a specific area of one colour on the image you want to play around with. If you just want to fill it with a block colour then select the paint bucket tool, or press 'g' for a shortcut. Then click on the colour blocks at the bottom of the tool bar (window>tools if its not showing) and pick a colour from the screen there. Then click on the part of the image you want to change the colour of. Press ctrl+d to deselect the area and see your results. File>Save As to save your image afterwards.

See:

http://www.ehow.com/how_8104_add-background-color.html

A tutorial shwoing a way to cut round complex objects and fill.

http://silurus.acnatsci.org/ACSI/corresp/digital_imaging_tips2.html

I don't believe the Color Replacement Tools is suited to this.

I suggest you try this: use the Eliminate White filter to make white areas transparent, on a copy of the layer (Ctrl+J). (You may find that applying Layer > Matting > Remove White Matte after this is beneficial). Then, you can paint or fill in what you want on a new layer behind this one. If you used Remove White Matte a solid white fill on the back layer should give you the original layer appearance.