Now that we've done some brainstorming, it's time to see how it turns into a game.
Since Rainbow-signers embody change, that will probably lead in to our magic system (because every game has to have things players can't do in real life, whether it's psychic powers, magic, or the ability to pull of really crazy stunts). But I've established Rainbows as pariahs, so it should be as much a curse as it is a blessing. And since it's about change, there are no "something from nothing" effects, like fireballs or pulling rabbits out of hats. It should play a little bit more like manipulating luck, creating blessing or curses.
What comes to mind is a boiler. If you just let it boil, it will continue to build pressure until it explodes. So you have to keep an eye on it, releasing pressure as needed to make sure that doesn't happen. Turning this into a game mechanic, we have a number (call it Change) that continually rises as game time passes. 1 point per day, let's say. When the player wants to cause a change, they roll a d10 (because the contest rules say I have to use d10s) and adds the die roll and the current Change score together. If this total is 10 (a nice round number) or higher, the effect happens and pressure is relieved (Change points are reduced).
Now for the curse part. Anytime a player rolls a natural 1 (a 1 is showing on the d10) on any die roll, they must roll a Chaos check. This is just like the Change roll, adding a ten-sided die to the current Change score and seeing if the result is 10 or higher. But if this check is successful, it bleeds off a number of Change points equal to the roll result -10 (so a roll of 12 would bleed off 2 Change points). These Change points then manifest in a manner determined by the GM (spoiling milk, turn straw into gold, and so on depending on the amount of Change).
This also does a nice job of suggesting a core mechanic. Roll 1d10, add stat, get 10 or higher to succeed.
But what stats will characters have? How does the rest of the weather-based astrology fit into this? It took some Mountain Dew-induced insomnia to answer that one.
Weather is made of three basic elements: Rain, Sun, and Wind. A typical day's weather consists of some measure of each of these things. There are days where the clouds and rain keep the sun away, or that are perfectly sunny without a could in the sky, but these are more exceptions that rule.
Thinking metaphorically, someone with a lot of Sun-aspect would be very noticeable and vibrant. But also, they would have a hard time avoiding being noticed, even if they were trying to stay hidden. So how sunny it was on the day of their birth would add to social ability, but subtract from stealth actions.
Extending this idea to the other elements, Rain would add to persistence, as rain eventually erodes everything that it falls on, but penalizes perception, as rain is indiscriminate. Wind aids movement, but spends so much time acting that it doesn't think.
To chart it out:
Rain helps Endurance and hinders Perception
Sun helps Presence and hinders Stealth
Wind helps Athletics and hinders Intellect
Sounds like the Basic Skills I used in Adventures in Oz, doesn't it? That's at least partly intentional. As much as I want the mechanics to support my themes, it needs a certain level of robustness. The game needs to be able to support a variety of actions that players might attempt. The AiO list was short and sweet and meshed with my current project in at least a few points and only minor stretching was required to make the rest fit.
The big thing we can say about this list is that it's not totally fair. Some things are helped and some things are hindered, but there's no element that helps the hindered things or hinders the helped things. I'm sure if I took a more in-depth approach, I could come up with a interlocking trait scheme, but it might not be as robust or have a satisfactory payoff in terms of serving my themes. We'll see.