How does QE effect the collaterals? For instance, the ECB accepts green bonds as collateral. And also they are more likely to include green bonds in a QE program.
Does this face make a green bond more valuable? How would it work in a real life example?
Yes, if green bonds are accepted as collateral they become more valuable/demanded by prospective buyers. When ECB directly buys these as part of QE then again they are more in demand. When demand shifts to the right and supply stays the same price increases.
In real life, ECB could just buy these green bonds on the market as part of their QE, for example, they could directly buy Enel Green Bonds, that increase in demand will raise their prices so when Enel issues new Green Bonds they can charge more for them than they face value.