Something that came to mind: If you're doing a 'blood' series or element, something that could balance it, is 'blood pillars' that work by reducing the HP cost of any 'blood' element card by 1. I figure that to prohibit the use of this in rainbows, the HP costs would be rather significant. Ex: something like say, a Gnome Rider would cost maybe 5-7 HP, meaning that playing blood cards without blood pillars will suck you dry very quickly.
I'm curious what you have in mind Emerald...Something I like the idea of is that the 'blood' element is fairly powerful, to compensate for the sheer speed gained by casting from HP.