What filter? Modern vertebrate blood is always red (with just one exception AFAIK: https://en.wikipedia.org/wiki/Channichthyidae ) because it is based on hemoglobin. Given that birds, which descend from one branch of the dinosaurs, have red blood, it's likely that the dinosaurs did as well. Likewise plants are mostly green due to the chlorophyll used for ...
The main reasons the atmospheric oxygen remains constant are:
The sheer quantity of it. Not even a large forest fire will
measurably deplete it. Most of it is fossil oxygen created many
millions of years ago by photosynthetic organisms.
It is constantly being replenished by photosynthetic organisms,
mainly in the ocean.
Measurements of the percentage of ...