OCOs of Bank 7 (Nature FX) are implemented as a color change.
Most of them are straightforward.
To make the color picker appear, set Bank to 7 in the CO tab, then set Object and choose a color.
With Rain (7:8 and 9) and the Purple Leaf (7:6 and 7), things get messy.
For example, if you make rain yellow (255,255,0), the smallest particles will be white.
If you make the purple leaf magenta (255,0,255), some of the leaves will be white, and some black.
The reason this happens is because the internal code makes the smallest raindrops darker than the others.
Similarly, the Purple Leaf has three different particle variants.
One of them is the color you choose, another is darker, and the other is brighter.
Now, the 0 in yellow is already the minimum, so the code makes the B value "do a 360" and end up on the opposite side of the range (255), then subtracts from there to make it "darker"...
That's why rain turns white.
Worse things happen to magenta in the purple leaf.
Both of the 255 values (R and B) are already max, so they become 0 and add from there, turning the "bright" leaves black.
While the dark leaves do the opposite (what rain does), so they turn white.
In order to avoid this "cursed" behavior, all you have to do is stay within a certain range.
The darkest color for rain is 15,18,19, while the brightest is white (255,255,255).
And yes, that means white rain works fine of course.
So you just need to pretend 15, 18, and 19 are the minimum values of their respective channels, as if they're 0.
If yellow is 255,255,19, it stops turning white.
Other examples:
Brightest red rain = 255,18,19
Brightest green rain = 15,255,19
Brightest blue rain = 15,18,255
The Purple Leaf has a max limit too.
The darkest color is 23,16,30, and the brightest is 235,231,240.
So if magenta is 235,16,240, it stops being silly.
Other examples:
Brightest red = 235,16,30
Brightest green = 23,231,30
Brightest blue = 23,16,240
Thanks to Robin (Pie Sniper) for helping me figure out these Fusion shenanigans