We can achieve a smooth transition from one metal to another by blend gilding. It gives the look of one colour gradually changing into a different colour.
This picture of a rainbow serpent has 6 different metals, blending from red gold to silver - 23.5ct, 23ct, 18ct, 16ct, 12ct gold and silver leaf.
It works best with water gilding, abrading oil gilded work produces a dull result. You can blend several different metals in a line to produce a rainbow effect. I find it works best to use the thinner metals (gold alloys) for the first stage and leave the more robust metals (silver,platinum, copper etc..) For the second backing metal.
To blend from matt gold to bright gold, you have a choice of two methods.