For me, the 20% bonus for feeding on color trumps the need for immediate results with one hero as a general rule. But that’s just 'cuz I’m good with patience, and hate inefficiency at a personal level. Most of you probably aren’t like me!
For a new player working on their first two rainbow teams of level 70 heroes, this is very strongly true.
For a player who already has 40 level 70+ heroes, power leveling one or at most 2-3 heroes at once makes a lot more sense, especially if those heroes are ones that will make a real difference on raid defense, against titans, or raid offense, instead of just being another war filler hero.
For the many players inbetween those extremes, you kinda pick what you like / what works for you.
As a side note, I’ve been surprised to find how much I like using my second Grimm, although as soon as I get and max a second blue 5* that works on raid offense, that’s probably going to end. The Grimm / Drake / Alasie / Delilah / Grimm raid team is pretty amazing for me against red tanks.