The unlocks rewarded at the end of each mission are intended to be used throughout later missions, and should be fairly straight forward to select.
Since I played this game on console, I reference an online copy of the game manual, intended for the Steam version of the game.
How to equip equipment and weapons
To access the inventory of weapons and gadgets, press and hold the inventory key. Use
the mouse or the directional keys to select the desired piece of equipment; release the inventory key to confirm. Press E to equip or de-equip the selected weapon.
If you can see the equipment in this inventory list, you have it unlocked, and should be able to freely access it. If you are still having problems selecting it, it is possible that you are experiencing a bug or a glitch.
How to equip vision goggles
You have access to three differant vision modifiers in the game, one of which is night vision. IIRC, wave vision appears in colour, and unlocks at a later point in the game. It is possible that by "colored night vision", you are actually referring to thermal. Regardless, switching between these options is pretty straight forward.
- Heat Vision: Press number key 2 to activate heat vision.
- Night Vision: Press number key 1 to activate night vision.
- Wave Vision: Press number key 3 to activate wave vision, a mode that allows Sam to quickly spot all electricity-using objects.