You can also purchase a Blue Keycard at an Outpost for 100 Scrap if you want to do that. These Blue Cards can be found on Scientists or through the Green Doors in Harbor, Satellite Dish, and Sewer Branch.

Blue Doors/Blue Keycards Source: Rust Wikiīlue Doors are the medium puzzles in the game because it has the perfect loot for an average player. Basically, if you wear a Radiation suit the entire time you do any Monument Puzzle in the entire game, then you will be safe from Radiation, but you should keep some Radiation Pills and Healing items just in case anything happens. The puzzles are very simple to get into the Green Doors and is a good way to get some decent loot in the beginning of your adventure. You can also find one on a Scientist if you can take one out at a smaller Monument. There are a total of five Monuments that you can find these Green Keycards and they are the Lighthouse, Oxum’s Gas Station, Junkyard, Abandoned Supermarket, and Abandoned Cabins. Green Doors are the easiest to get into and they give you some items that help in getting you into the next tiered door. Green Doors/Green Keycards Source: Rust Wiki

Even though the Red doors are very hard to get into, the loot you get is totally worth your time and effort in the end. Not only are you running around in a very high radiation area, but you are also in an area that has many Scientists that are trying to kill you at the same time. The Red Monument Puzzles are the hardest to complete because they are the biggest puzzles to complete in the game. It is a lot harder than Green because there is more stuff you have to do for it. It consists of turning on more power and Fuse switches so that you can get the door open. The Blue Monument Puzzles are kind of hard to complete because the area in which you must complete the puzzle is a lot bigger than normal. You can then go and open the Green door with your Keycard. The Green Monument Puzzles are the easiest to complete but require you to flip a few switches around the Monument and then place a Fuse inside a Fuse Box. These puzzles need to be completed before you can use any Keycard on a door, but it is not that easy to complete.

Why should I take the time to get into these doors?.

How many Monuments have these locked doors?.

Where to get blue keycard rust