So Red cores improve Weapon Damage, Blue cores improve Armor, but Yellow cores only ever give you 1 skill tier. Why aren't Yellow cores based on an attribute like the others, the most obvious one being Skill Haste?

Next, skills could have their specific tier based on a combination of Yellow/Blue cores (like Bulwark Shield, repair skills could be added there too) or Yellow/Red cores (there is no such example yet, but any offensive skills would make sense). There could also still be full Yellow cores skills (status effect ones?).