It's a great idea, but it doesn't really seem like "giving 'Mono' a boost" in the end. It's more like giving the whole Elements game play a boost, which is great. As Yalidilae said, though, it would make sense if it only came into effect while owning a pillar or tower (or two even) of the same elemental mark if it's to boost Mono. Or maybe something even more limiting like having at least half of your deck of the same element as your elemental mark to get the effect to work. Seeing as Rainbow decks have the advantage of amazing versatility, I think it would make sense to have at least half the deck quantity the same as your elemental mark in order for it to take effect. I think then, it would justify some of those powerful effects you posted. Because having a Water mark without any kinks to it makes Scorpio useless as a FG as an example.
![Smiley :)](https://elementscommunity.org/forum/Smileys/solosmileys/smiley.gif)
(Not that I've seen any water mark god farming decks before, though..) But, once again, I have to say it's a great idea.
More thoughts about the effects in particular:
Entropy: I think it fits in nicely, making Mutations more and more fun and efficient to play.
Death: Seems like it works, seeing as there's Afflatoxin and and Boneyard/Graveyard. Sacrifice a Malignant cell then sacrifice an Elite Skeleton or two, make Bone Walls pretty impenetrable through normal means.. seeing as you have to tear down each of its counters one by one. Might need some work on this one.
Gravity: I like this idea. Seeing as Otyughs are widely played, it would be nice if the gravity inclined player would have a small advantage over the Rainbow inclined player.
Earth: I don't really like this effect much, as I see it as being a bit weak in comparison to the others. Probably an innate damage reduction of 1 would be more suitable.
Life: Seems okay, but Life already has Feral Bond and Heals. So, perhaps this one should have the "All of your creatures heal 1HP every turn" effect. Life should thrive with this mark, right?
Fire: This would be really nice. Seeing as there is a trend for low HP fire creatures with high attack.
Water: Limited and not the best idea as stated. I'm not sure what's more suitable though.. perhaps your opponent's creature's freeze/congeal for an extra turn?
Light: I think this element would use the increased max HP effect more efficiently than for Earth. 120 HP would be a lot, considering how the person can use miracles.
Air: I think it would probably make more sense to do 1 damage for each
![Air :air](https://elementscommunity.org/forum/Smileys/solosmileys/../../../images/Misc/air18x18.png)
creature you have in play, not just any.
Time: No suggestions here, it's great.
Darkness: The effect doesn't seem that great, really. It would have quite an effect if you had an abundant amount of dark quanta saved up to summon several creatures, but it seems a bit limited overall, though. I'm not sure what to suggest for this, though..
Aether: No suggestions here, I think having a another face for immortal creatures is interesting.