In designing some pharaoh decks, I encountered the need to put special emphasis on certain elements. The deck still had to be rainbow, but also a 3 color element deck in a way. That and later experience has brought me to what I am presenting you.
This is an example of a duo-element focused rainbow (it is rainbow but focuses in two elements)
5rp 5rp 5rp 5rp 5rp 5rp 6qq 6qq 6qq 6qq 6qq 6qq 6qq 6qq 6qq 710 710 710 710 717 719 719 71b 71b 71k 71k 74b 77i 77i 7am 7am 7q0 7q0 7q0 7q0 7q0 7q0 7q0 7q5 7q5 7q5 7q5 7q5 7q5 7q7 7q8 80b 80h 80h 80h
First take note of the quantum tower counts
7 time tower (+ time mark)
4 death tower
9 quantum tower
Total of 20 out of 50 cards, or 40%.
Then take note of the card makeup
8 time cards (14 including sundials)
7 death cards
9 "other" cards
The 7 death cards and 4 death towers could be replaced by any other element. The 9 "other" cards can be be switched around as you wish. Having special focus on additional elements (death + light for example) is feasible, although more prone to bad luck. The time cards, however must stay relatively stable, as hourglasses and sundials are the engine for the deck. They save time so quantum towers can build up more, and so you can get whatever cards you need. I'm not sure if element-focused decks could work well, without having Time as its backbone.
I am not sure if the balance of 7 Time towers + Mark, 9 quantum towers, and 4 X towers is correct. And of course it could change a little from deck to deck. But I think it is very close to giving you the quantum you need to specialize a rainbow in certain elements.
I think element-specialized rainbows may be the key to doing better against FGs. I'm not sure if standard entropy or time rainbow will do the trick, as the combination of cards you can pick from is limited. Focusing on elements opens new strategic possibilities.
I hope to see some people experiment with element-focused rainbows!
Disclaimer: This deck was made in the trainer, and is not really functional yet ;p Hence no shards, and 2 death nymphs (I only own 1)