To survive, you don't need to put all the colour you gather into all of your hearts - just having one heart filled at a time is enough. So don't put everything from your memory into your body at once! If you're afraid of dying too early, just monitor the one filled heart, and fill another only when the first one is getting empty.
Remember that refining all the colour as fast as you can to paint as many trees as possible early might not be the best tactic - try for example refining enough colour to paint less trees, but keep enough to survive for the next of the cycle; also Amber speeds up the process of refining, so if you're afraid you're losing colour too fast, put less Amber into your hearts.
If you want to explore the rest of the map, you will need to feed the Sister with colour using the donor glyph to get her to open the passages to adjacent chambers.
Yes, there are some trees in Fireflies that can be coloured too. Remember you can also hunt the little floating critters - drop some colour on the ground with donor when a firefly's near, back off, wait for it to get take the bait, then grab it when it's feeding on the colour you dropped. That's also a trick that should help you survive if you have colour in your palette but not memory - drop some on the ground using the donor glyph (that way you won't lose precious drops) in the air, then collect it again - it's put it back in your memory, so you can fill your hearts with it again. (It probably does some domain damage, but hey, if you were going to die otherwise that shouldn't be that much of a concern.)
Hope any of this is helpful, and that you get to enjoy this great game.
