Uranus and Neptune are more bluer than Jupiter and Saturn distinctively because they contain more methane within their atmosphere. Uranus & Neptune contain no liquid metallic hydrogen and their lower masses do not raise the high internal pressures required for the conversion of liquid molecular hydrogen (which they do have) to the metallic form of hydrogen. Both Jupiter & Saturn contain both liquid molecular and metallic hydrogen. Both Uranus & Neptune do contain slushy zones, which are absent in Jupiter & Saturn. These difference most likely occurred due to the each planets distances from the sun; For the heat affects each planet differently and the elements, colors etc. within each planet.
