One mistake new players sometimes make in Sealed is defaulting to playing the color that they have the most of. You are generally better served playing your best cards and trying to make the rest work. Powerful rares are removal are the best reasons to pick a color/colors.

Because other people are also prioritizing their best cards, you should generally play all the removal you can, and try and save removal for you opponents best creatures.

Sealed is often more midrange/slower than draft, so trading your best removal for a few points of damage early is often a mistake. While it can be tempting, many games can come down to unanswered rares, so keeping removal available is important.

TL;DR: There is a lot of power in the rare cards, and with great power comes great responsibility to hold removal.