I've created a short guide on Flowerpot Colors and how to grow plants to a specific color. You can also find this under the Resources link in the navigation menu.
We're already seeing some results from the Flowerpot fertilizers. When planting an Oldrose seed it will always start off with red color. Using different color fertilizer even once seems to change the color. Not sure how much this affects the end result yet. Here's some current info I've gathered:
Blue x1 = Green
Red x1 + Blue x1 = Purple
Blue x1 + Mountain x1 = Green
Red x1 + Mountain x1 = Orange
Red x1 + Mountain x2 = Yellow
Red x1 + Blue x1 + Mountain x1 = Red / Unusual Shade
Oldrose seeds are the only seed that cannot be used in an outside garden plot.
Any seed can be planted in a flowerpot and fertilized with the new pomace fertilizer.
The elemental seeds (Firelight, Icelight, etc...) appear to be gathered by Botanists.
I've added a couple new tabs (Flowerpot & New Crosses) to the Gardening Spreadsheet (yes, that one!) to track info on the new crosses and color changes. I will be adding results from trusted sources to these two new tabs until I have enough info to incorporate it into the site.
I've put together a page to keep track of all the updates coming to gardening in 3.3. I've pulled the info out related to gardening from the patch notes and plan to update this page as results start coming in from the community next week.
I will eventually organize everything into its own separate Flowerpot page and will take it from there.
Feel free to post your results through Facebook or even the Jumbo Thread on the official forums. I might create a new spreadsheet to gather and share the data before putting stuff into the database.