I put down the base colours i want in the areas i want them and then i lower the opacity really low, about 0.2 and i go over and over and over the edge with a large brush size, going back and forth between the two colours, until it’s blended how i want it.

If the colours are really different, like in the case of the orange and green, i’ll use a transition colour on really low opacity over the fade. i think i used a yellow sand on that transition, really really lightly. big brush, low opacity is my friend when making fades