Her biography really explains how sorry she is

Well, if we stop, take a step back, and look at all the causes and effects all the Diamonds had on each other thought the series.

Blue and Yellow could be forgiven since they were basically doing what White wanted out of fear.
Blue never really called about universal empire domination and tried to at least keep some alien faune and flora alive before ending all live in their planets.
And Yellow was more of a general doing their job but if White gave the order to stop then she would stop and not really question much since the Empire was more for White then for herself.

So forgiveness and redemntion works with these two since at their core they never really wanted to do evil. They simply were told to do it by Giant Space Lady Gaga that can hijack their bodies and minds by staring that them if she wanted.