Never boil pre-hopped extracts,dry or liquid! It converts the flavor & aroma hops to bittering,ruining the flavor profile. Not to mention,the LME style cans will also darken & get that twang from mailard reactions. I use the plain DME in the boil for hop additions,adding the remaining extracts at flame out.
This gets rid of the twang,giving better flavor & keeps the color lighter as intended.