Can't change biome preview color in Dimension Generator

[Sniper1-1]

When trying to create a custom dimension I could swear you used to be able to change what color the biomes would show up as in the preview window. This was helpful to get a good visual of how the layout would look, but now the color seems to be randomly chosen and can't be changed. It makes it difficult to visualize as I have a biome that's very green and one that's very gray but now the gray one shows as green and the green is some kind of blue. I have to hover over the different colors in the preview to see what's what and then remember that as I change things so that I don't confuse which biome is growing/shrinking. This gets complicated with a lot of biomes added and in 3D space.