Orange seeds grow paths in a straight line, and don't overlap, usually to the right or left of blocks, straight up if there is nowhere else to go.
Green seeds grow like trees and don't overlap plus they grow hearts like fruit, more on this later.
Purple seeds grow in small patterns, fairly uniform without any overlap.
Light seeds blue grow into cross shapes, that overlap other blocks
Dark blue seeds grow a single block that increases gravity.
Dark yellow seeds grow a set of blocks that go downwards as much as they can, and they increase gravity.
Pink seeds grow slowly, and only upwards, and if you destroy them you get a new seed.
Red seeds grow a single block, that if combined with another block explode into a big block roughly 7 across and down, with overlap. If you are caught in the radius you are shifted to the alternate world.

Now, the alternate (negative?) world is already around, right at the start of the level, and it begins eating away at the stage. You have to grow your seeds to find stars in negative space.

Beware, when you touch them, you go into negative space and if they aren't connected to another block, you'll be stuck and have to restart. If any of your blocks touch a star block, they'll begin to erode.

So far what i've figured out is that there are lots of stars that you need to connect to get keys. If your green blocks (with heart plants) turn negative, the hearts can be collected. Those seeds can then be used when you go back to the hub... or you can save them for the start of that stage.

Other than that i'm clueless. I've seen a 3 key lock, but not managed to get three keys.

I've been growing stuff at the hub, but I don't know why.

Looks dark yellow to me, is weird. They decay slower than others too.

What you've missed about the key is that it's purpose IS to send you back to the start, BUT any hearts you've collected remain as seeds. If you don't use the key you have to press H and you loose any seeds.