Nine lines of text! I think this is the limit for Magic: The Gathering cards to be able to be printed. Or was it eight? But I couldn't think of another way of wording this correctly. What actually happens is quite simple - it's just a lot of templating text.
So, what's happening here is that there's a screw counter on one of the colors. Each upkeep, the screw counter moves clockwise onto the next color. While the screw counter is on a color, mana of that color can't be added to mana pools.
I initially was going to go for making all color of mana that would be added to mana pools of that color to become colorless instead, but I wanted to make sure that Mana Screw had a strong enough impact. For example, a white-blue deck could decide to play their blue spell during a turn in which white was currently being "screwed." Then they play their white cards after the screw counter moves. Not much impact, thus, the effect I chose on the final card above.
Since this effect can be powerful, I initially thought to make this a higher-costing card. I have no idea how to cost this card, though. I thought "5" was a nice cost due to referencing the five colors of mana. However, I realized that in order for Mana Screw to really have its full impact, it would need plenty of turns for the screw to move around. Thus, I needed it to cost lower, so it can be played during earlier turns.
I had the screw counter enter the battlefield on a color of an opponent's choice because I didn't want this card to act like a Time Walk for you if you're against a mono-colored deck, and you ensure that their next turn would be completely devoid of any action. This is bad if it's turn two, and they can't do anything.
Besides, I figure that the player who decides to use Mana Screw in their deck would be prepared to handle Mana Screw's effects.
If you look closely, you'll notice that the rules text implies that there would ever be multiple screw counters on Mana Screw. I did this because other cards in Magic: The Gathering DO have the ability to put more screw counters on Mana Screw. Like Gilder Bairn.
As for the ruling on how to handle putting more screw counters on Mana Screw, we'll leave it to Mark Rosewater to handle the ruling (he handles rulings for Un sets). Don't worry, this is normal with Unglued and Unhinged cards. Some cards are just too zany!
Speaking of which: if you're at all confused by the silliness of this card's design - there are two sets in Magic: The Gathering's existence that are "silly" sets full of cards not meant for tournament play. They poke fun at various aspects of Magic and enable funny gameplay. With that in mind, when designing a card for an Un set, it must do something that "breaks the rules" of Magic - it should be a design that couldn't normally exist in "normal" Magic: The Gathering.
I want you to notice that the original design of Mana Screw didn't have anything to do with colored mana at all. Yet, there are colors within Mana Screw's art. My design made sure to utilize these colors.
Lastly, and I'm most proud of this: the fact that the screw counter goes 'round clockwise is reminiscent of what a screw does - when you spin it clockwise, it screws down.