Are these their “official” color schemes, or did you come up with them yourself?
The Space Marines are a colour called 'Hawk Turquoise' and they are a completely unofficial (ie I made them up

) chapter called the 'Salamanes'.
The Tyranids are Hive Fleet Leviathan, which is stated in the lore to be one of the most brutal enemies in the entire Imperium, so there are a lot of humans and aliens trying to stop them.
The Imperial Guard are the Carpathian 63rd, and they were annialated by Hive Fleet Leviathan in recent lore. I thought it was kind of cool making an army that was already dead and gone, so that every game I play is sort of a 'flashback' to prior to their destruction.
More pictures:

A Meltagun Command Squad in front of a Chimera APC.

The stages of the painting process from bare metal (the far left) to black undercoat (centre) to fully painted on the right.

An Imperial Guard Medic. If you assign him to your Command Squad, it doubles their chances of survival against most weapons.

A Tyranid Lictor, the stealthy assassin that is expensive and limited selection (3 Maximum per Army) but makes up for it by being able to appear anywhere on the board!

A Tyranid Zoanthrope, the 'spellcaster' unit for that race. They are expensive and weak, but have very VERY powerful psychic shooting attacks and a powerful shield that protects them from almost all harm.