RMVX ACE

Lower the drop rates for the items, I suppose, or give it out in very specific conditions. Basically, how much you decide to give out will decide the rarity in a sense

Or have I missed your point entirely?

I'm having a hard time trying to follow exactly what it is you're trying to do. Are you trying to make it where when you open a specific chest, you have a chance to get one of four different items - with some being rarer to appear than others? For example: When you open said chest, you have a chance for;
Potion - 50%
Hi-Potion - 30%
Hi-Ether - 15%
Elixir - 5%

If this is what you're looking for, all you need to do is work some randomized variables and conditional branches related to each. The other possibility of what I'm seeing you wanting to do is to have it where an item that is 'rare' is designated by different color in the inventory list? That I'm not 100% sure how to do, I know there is a code to change the color of text in dialogue, but I am not sure if it works for inventory items.

All you have to do is put

<Rarity: rarity> (replace rarity with Common, Uncommon, or whatever's defined in the script)

in the item's note box to change the colors. It pretty much spells it out on lines 13-17

Ok! This is relatively easy to do using variables, and trust me learning variables will change your RPG Making experience. To create the simple randomly generated chest you're looking for, create a new event to represent your chest. Have it be activated however you wish, but in the event field begin with this:

On Page 1 under Game Progression, hit Control Variables.
You'll use a single variable (this will also be the only variable you need for all random chests you need in the game!) And name a variable 'Rando Generator' or whatever you want to call it. Operation should remain 'Set', and the Operand should be 'Random' with the two data boxes being 1 and 100.

What all this means is that when you hit this spot in the event, the game will generate a random number between 1 and 100 and store it in the variable you named.

Now for the complicated looking part. The next line of event should be on Page 1 under Flow Control. Hit Conditional Branch.
Now you'll see a conditional branch detail with 4 pages. What you'll need is on page 1, the button labeled 'Variable'. Select the variable that you had the previous line record your randomly generated number to. Among the options below that select 'Less than or Equal to' then below that you want to use the 'Constant' option, and set it to the percentage chance you want your COMMON item to be. (for example, 50 to represent the 50% chance for Potion) Make sure you leave the 'Set handling when conditions do not apply' checked.

Now what this does is fork your event. Under the 'Conditional Branch' line you should see a line that is simply @>. Then below that you'll see 'Else' and another @> following that. IF your randomly generated variable is indeed equal to or below 50 (Representing 1% to 50%) it will continue events on the first '@>'. If they are not less than or equal 50 (the remainder of numbers above 50, showing that the random generator hit a rarer number) then the event will continue on the second '@>' under 'Else'. So under the first @>, this means that the generator had a number 50 or below so put whatever events here you need to get your player your common 'Potion'. As for the second @>, you will need to start a new Conditional Branch.

This second conditional branch will be almost identical to the first, so you can copy/paste it here. Then right click to edit it, and change the 'Constant' from 50 to 80. (To represent the 30% chance that isn't 1 to 50!) Then modify the rewards under the first @> to give the UNCOMMON item (Hi-Potion in this case.) You can repeat this process for as many different items you want to use, but we'll stick with the original four.

Copy/paste the Conditional Branch below this new 'Else' field one last time, and change the 'Constant' from 80 to 95. (To represent the 15% chance that isn't 1 to 80!) After this, there is one minor addition.

Since you're up to the last item, you don't need an additional Conditional Branch. The last 'Else' option only needs you getting the rarest item (Since you don't need to check and see if your random number is rarer than the rarest item.)

Then after all that mess, you can finish the event. Just do whatever you need to do to get to the next page after the very last 'Branch End', usually a self switch is all you need.

Since this is probably pretty complicated to understand, I took a picture of what the event should look like that you're building. The second page listed is to represent when players try to access the chest again once it's been opened.

Best part is, you can copy/paste this event and just change the items you receive and it will work perfectly fine no matter how many you have.