(* img is the original image *)
bg = Binarize[RemoveBackground@img, 0.01];
colorbg = Graphics[{
    RGBColor@paleblue, Rectangle[{-1, -1}, {0, 1}],
    RGBColor@red, Rectangle[{0, -1}, {1, 1}]
    }, Method -> {"ShrinkWrap" -> True},
   ImageSize -> ImageDimensions[img]];
i5 = ImageAdd[ImageMultiply[i4, bg], ImageMultiply[ColorNegate@bg, colorbg]];