Potions are liquids of various colors that players can use to aid them in their travels, or sometimes to hurt themselves. The color can usually indicate what the potion will do. For example, red usually heals, either a set amount of hit points or else a percentage of total health. Blue usually restores mana points in the same way. Other potions have buffing effects such as increased attributes, damage, damage resistance, temporary immunity to poison or disease, and so forth. Some potions heal poison and disease; they are sometimes white. Many RPG games have potions called elixirs that heal all health, mana, and cure all negative effects and afflictions, but may not be called elixirs. In many games they sparkle or have some other way to note that they are special.
Some potions may have unknown effects until they are tried, in which case they may help in either good or bad ways.
In some games, such as Diablo 2, you may throw exploding potions that can damage an area of enemies with the elements of nature, poison, or debuffing effects.
Not all potions have colors, in some turn based RPGs they may just be named and you use them without seeing them.