Parameters

[potiontype] - The type of potion to create. 1 is normal, 2 is splash, and 3 is lingering.
[effects] - The effect(s) to add to the potion. Can be multiple if separated by a comma (ex: strength,speed).

Optional Parameters

[seconds/infinite] - Defines the time to give the effect(s). Default is 30s, max 2,147,483,646.
[amplifier] - The amplifier to give to the effect(s), max 127.
[showParticles] - Defines if the potion should show the potion particles when applied.
[customName] - Simply gives the potion a name, works the same way as re-naming something with an Anvil.
[R] [G] [B] - Changes the color of the potion to the given RGB value.

You can make custom potions without using this specific command, this just makes potion-making simpler.
Requires cheats: yes.

5.5.0: Introduced command.
5.5.2: Improved command, allowing effects from other (forge) mods to be used with potions.
5.5.3: Decreased amplifier cap from 255 -> 127. (byte value moment)
5.5.3: Namespaces for effects are no longer required.
5.5.4: Decreased amplifier cap from 127 -> 124.
6.0.0: if a custom name isn't specified, the name of the potion given will be based on the effects (using potion name if possible).
7.0.0: Removed the RGB command args because the current neoforge version doesn't have an easy way to set custom potion colors.
9.0.0: Re-added the RGB command args.