Good evening!
First, a disclaimer - The Void is a very unforgiving game and is meant to be so. BUT, be strong, and you'll understand why and be rewarded greatly
- You can't change a color of the tree
- You can't add color until it's shed it's blossoms
Try to look for small trees in Fireflies - there are three, one a bit tricky to find.
Try to "load" new trees to the maximum.
Don't worry, if you loose some time at the very beginning — just build up your power gradually. Maybe it's best to revive a lot of trees with max Color before feeding the Sister.
Did she say anything? If she did it's ok - you need to upgrade her one more time.

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How happy is the blameless vestal's lot! The world forgetting, by the world forgot.
Eternal sunshine of the spotless mind! Each pray'r accepted, and each wish resign'd;
---- Alexander Pope