I have a design in bitmap format that has a grey to blue gradient. Can you change the blue and shades of blue to reds?
Maybe... if you can select the area that you want to affect --- either with the magic wand or drawing tool. It doesn't necessarily need to be very accurate depending on what colors border the gray/blue gradation.
Then try: Image > Adjustments > Replace Color...
A menu will open > with the eyedropper tool (left one) sample the color blue you want to change. I'd probably choose something to the medium dark range.
There will be a box that shows the level of Fuzziness --- adjust from 0 to 200 for your range of blue to change. Then adjust the Hue, Saturation and Lightness to your preference.
Hope that makes sense.