Diablo 4 only lets you carry one Liquid Rainbow, so you should probably drink it
The consumable opens Whimsyshire, and keeping one can block you from picking up the next copy that drops.
If you have a 
Liquid Rainbow in your inventory in Diablo 4, go ahead and drink it. You can only hold one at a time; it cannot go in your stash, and there is no hidden recipe waiting for it. Saving it does nothing except block the next one from getting into your bags.
We wondered about this ourselves, so here is what the item does, what it does not do, and a few things players have learned the hard way.
Liquid Rainbow Opens Whimsyshire
The Liquid Rainbow is a Unique Consumable item found during Diablo 4 Season 15. Drinking it opens a rainbow portal to Whimsyshire right where you are standing. Portals can also appear at random, and we went through every way into Whimsyshire separately. But Liquid Rainbow is the only way to get in on purpose.
You can get one from Season Rank V (5), and you do not need to combine it with anything in the Horadric Cube or collect any other ingredients first. Simply use it from your Consumables inventory. If you are trying to get another Liquid Rainbow, the item can also drop randomly (like while leveling, during the campaign, in Baal capstone runs, and so on), but it is pretty rare.
Just do not expect to be able to stack them. You can only carry one per character, so if a second one drops, you have to drink the first before you can pick it up.
What Even Is Whimsyshire?
If Whimsyshire is new to you, it is the secret joke level from Diablo 3, full of rainbows, unicorns, and very angry teddy bears. Season 15 brought it back as part of the anniversary celebrations. Apparently, the devs created the original Whimsyshire (in Diablo 3) as a funny response to players complaining that Diablo 3 was not dark enough.
Reset the Dungeon Between Party Runs
If multiple people in your party have a Liquid Rainbow, do not use the next one immediately after completing Whimsyshire!
Without resetting the dungeon first, the next Liquid Rainbow can still be consumed, but its portal will lead your group back into the Whimsyshire instance you already cleared. That means no new enemies or loot, and one rare consumable gone for nothing.
Once everyone has left Whimsyshire, have the party leader reset the dungeon. The next player can then use their Liquid Rainbow to open a new run.
Make sure everyone is ready before opening it, too. As we covered separately, Whimsyshire portals can disappear rather quickly.
Clear Your Bags and Go In Fast
Clear your bags first. Once you leave Whimsyshire, you cannot get back in, so make room before you go.
Go in right away! The portal does not wait for you. Players report it can vanish within about 30 seconds if you do not step through.
Click the Happy Clouds. They are loot chests, and they drop a lot.
So What Is the Black Mushroom For?
The 
Black Mushroom is another returning item, originally from Diablo 1 and Diablo 3, but it currently has no confirmed use in Diablo 4 other than a small joke interaction.
In Diablo 3, the Black Mushroom and Liquid Rainbow were two of the ingredients needed to craft the Staff of Herding, which opened the original Whimsyshire portal. Because of that connection, it is only natural that players now hunt for another hidden recipe in Season 15. However, at the time of writing, the community has not found a working Staff of Herding recipe in Diablo 4. Consuming the Black Mushroom does nothing, and there is no known reward or gameplay benefit.
Keeping the mushroom in case another secret is discovered makes sense. But you do not need to save your Liquid Rainbow with it. The rainbow opens Whimsyshire all by itself, so if you have one, use it.