One possible hex color for a fictional gas giant with a higher presence of methane than Uranus could be #954C04. This color is a deep, brownish orange that represents the higher concentration of methane in the planet's atmosphere.

One possible hex color for a fictional gas giant with both methane and ammonia present could be #352D64. This color is a deep purplish blue that represents the combination of methane and ammonia in the planet's atmosphere.