Levelling heroes of the same colour feeds will give increased XP! Ultimately means you level your heroes faster with lot less ham (aka food.) As for levelling with 1 or up-to a max of 10 heroes per feed, feeding using maximum 10 per feed will increase the % likelihood of increasing special hero skill. Max level for any hero specials is 8, higher you get better the specials is, and these can be various abilities depending on the hero. Your aim is to have max specials before your hero gets into an ascension lockdown, which means you waiting for rare ascension items to ascend the hero to the next tier and continue levelling once again and making the hero even stronger. There are total of four ascension tiers and ideally…, one would want to have the specials maxed before the end of second tier on a 5* hero, third tier on a 4* hero.
Other things…, feeding with its double, gives you 25% chance increase for specials, and most experience! Using two doubles will give you 50% and so on…, but personally think it’s a waste to use more than one double per feedings.
Like BarryWuzHere stated, the tradeoff changes a lot from early game. For a while, working on my primary team…, I really had to go all out with the rainbow colour feedings! Several reasons, one being the hero roster cap limit! Either spend gems to increase it, or keep progressing in the game to have it increased naturally and enough to be beneficial to storing 10 on-colour heroes for several different coloured heroes.
I have done both rainbow and colour-specific hero feeds, and both types of feeds giving me successful special level increments to max state. However, there be times where we will be fingered and not have much success in maxing our hero specials before that dreadful ascension lockdown. Like my first and only hero… Wu Kong! Specials level 4 maxed at third tier!
Little more information…;
Hero feeding of the same elemental type; 1* +180, 2* +468, 3* +756, 4 +1080
Special Level Up Chance: 1 2%, 2* 4%, 3* 6%, 4* 8%, 5* 10%
Hero feeding of the different elemental types; 1* +150, 2* +390, 3* +640, 4* +900
Special Level Up Chance: 1* 1%, 2* 2%, 3* 3%, 4* 4%, 5* 5%
The above information is strictly based on using untrained heroes.