I finally figured what comes out of the rainbow adventures. I might be mistaken, but I've seen the pattern after going through several adventures. Here's the tip ... The type of coloured wisp you'll get depends on what you have in your storage at the time you collect from the adventure. So if you only have yellow, orange, and blue in your storage, expect to redeem one each of the yellow, orange, and blue wisps at the end of 4 hours.
At one time, I took the end of the rainbow adventure because I had everything except the green wisps. True enough, I collected all coloured wisps and the rock outcropping dust... but no green wisp! Another time, I only had an orange, yellow, and red in my storage. After the adventure, I also collected one each of orange, yellow, and red. I thought it was a random fluke, so I tried to go on several other adventures. Each time, the pattern continued. I'm not sure if this pattern will hold for other players, or those using other devices. But it doesn't harm you to try.
So I suggest, don't make your potions yet and keep a few of the wisps in your storage until an adventure is done. Might work... or might not. For me, it did. Hopefully, it works for you guys too.