Colouring up is a gambling term used in land-based casinos that describes a player exchanging smaller chip denominations for higher-valued chips.
Virtually every brick-and-mortar casino uses differently coloured chips, and each of the colours represents a different denomination. Thanks to this, both players and dealers have an easier time distinguishing between differently valued chips. Therefore, instead of having to check each chip individually for their value, dealers can do quick math and immediately know how much you have placed in a pot.
When a player is exchanging chips for a higher denomination, they are technically replacing the colour of their chips, ergo, colouring up.