Question

why does blood turn red when it hits oxygen?

Answer 1

Haemoglobin is the protein found in red blood cells, an it is what gives red blood cells their colour. Oxygen binds to haemoglobin and this is what gives red blood cells their colour. Oxygenated haemoglobin will give blood a bright red colour, deoxygenated haemoglobin gives blood a dull, dark red colour.

Answer 2

it's red because of the transition of electrons of the Fe atom are of the wavelength correspondent to the colour we perceive as red (crystal field theory). Electron transitions in d orbitals are such that they emit photons in the visible range. These are different when Fe is a bound to different atoms.