Quote from ilovesaprolings »I want the big art to see all the details.
This set is amazing
Quote from user_938036 »Each of your designs exemplify the problem with this mechanic. Its too easy to have the bonus for half your turn and then give it up after it wouldn't matter.
As long as you tap the mana dork first you will always gain its benefit, as long as you have 4 lands. Both creatures want to be attacking with their enlarged body and have little downside to shrinking in the second main phase.
For this mechanic to actually be interesting there have to be real decisions made. The easiest way to get the same flavor is to check at the end of your turn, this obviously encourages instant and flash cards as you can gain the benefit while still using mana, but at least you have introduced a limitation that is more than 'play the way most people play' and as long as its kept out of the more instant heavy colors it should be fine.
To put it another way. I'm fine with this ability. As long as armageddon or stone rain and ice storm are in the set, too.
One last note: keywording is not, inherently, a universal good. It's a tradeoff that sacrifices comprehensibility (Spectrum is quite roundabout) in exchange for...something. Self-referential mechanics need keywords, and other mechanics need to condense massive walls of text or appear a million and one times. Your ability doesn't fit under any of those, so what does this change bring to the table?
Quote from BlazingRagnarok »Eh, I'm not crazy about it. The wording that cares about colors spent actually has more design space than your idea. The former can fit into any multicolor set, while yours is pretty much restricted to color-matters sets. The level of parasitism is also a concern.