Keep them all for now. I will try to give you some info about game that will help you.
I would recomend to max 3 3* in every color before working on 4*. They are cheaper to max, will be usefull always in tournaments/events and will help you being usefull in wars even at lower level.
You need 30 heroes for war so keep all 3* for now untill you will have enough 4*/5* heores to replace them. Even without upgrading, heores can have some use, for example finishing enemy in war that has 1 hero with very low health left.
Green
I would max Belith, costumed Isshtak and Brienne (in this order). Carver isn’t worth upgrading and I wouldn’t upgrade Berden without costume.
Blue
Gunnar is great 3*, he can help you beat stuff that without him would be unbeatable.
Graymane is bad but since he is maxed he will be usefull for you anyway.
Costumed Karil is good 3*, you might want max 2 of him. Ulmer is also worth maxing.
Red
Hawkmoon and Namahage are both worth maxing, Jahangir isn’t anything crazy but he can be helpfull if you won’t pull anything better.
Purple
Chochin is solid hero, worth maxing. Personally I don’t like Gill-Ra and I’ve never used her so I can’t say much about her. Baltazzar is good 3* sniper.
Yellow
Dawa is bad, don’t upgrade her. Gan-Ju is pretty good, you can max him and he will be usefull.
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Very important thing is to find active alliance with people that talk and can/want to help you. Alliance is also one of the best way to get loot (from titans and war).
Another important thing, if you will get 5* don’t start working on it immediately. 5* are expensive and requires a lot of items to finish. They can wait untill you will finish 15 3* and at least 15 4*.
You can lock heores to avoid eating them by accident. Tap this padlock, if its white you can’t feed hero, you can undo this any time you want.