Colourhaxing refers to creators setting specific colours to be used for new combos, instead of cycling through the available colours in order. This allows for full control over combo colours and can be used to make different sections of a beatmap stand out.
A very common way of doing this is to have the kiai time use brighter combo colours than other sections to make it stand out even more. Colourhaxing can be a very tedious process, as new combos are defined through references to earlier combos. This means that changing one combo will change all of the following combos as well.