I have this dolphin and I only want to change the blue to another color. The eye and mouth are also painted on. Is there a way to adjust only the blue without painting over the eyes and white on the bottom?
There's no way to automatically replace a color, but to avoid a meticulous repaint you could put a Color > Hue/Saturation/Value behind the Image Texture node and tweak the values:
Or do the same in an image editor app like Photoshop or Krita. In Krita it's in Filter > Adjust > HSV (Hue/Saturation/Lightness):