Cosmo Balls

How do we get the "Yellow" ones?

I can fill the blue and red circles, but having trouble with the yellow one. NPC that sells Orange tier balls don't have yellow ones.

You need buy book for get Yellow star but here no 100% chance for get yellow you can get red or blue...
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Any idea why I can't equip 2 Yellow stars?

I have 3 of them now, all blue tier, but I can only equip 1, despite having 2 slots open.

Can't use the same type of "yellow" stars for example in your picture it says Shield which blocks 4000 damage. You can only equip one of those types.