Brand colour palette
A defined set of colours with fixed values and assigned jobs, not simply a set of swatches. A usable palette specifies each colour in every space it will be reproduced in, states which is primary, which are secondary and which are functional, and records the pairings that meet contrast requirements.
Brand colour palette is not Colour scheme.
A colour scheme is a pleasing combination chosen for one composition and re-chosen for the next. A palette is governed: fixed values, named roles and approved pairings that must hold across every future piece of work. The palette is precisely the thing that stops the scheme being re-picked.
In practice
A palette becomes usable at the point where every colour has both a value and a job. The value part is mechanical: an exact code for every space the brand appears in, which for most people means hex and RGB for screen, plus CMYK or a spot reference if anything is printed. The job part is where palettes usually fail. Five swatches with no assigned roles is a mood board, and it guarantees that whoever builds the next page will decide by eye which one is the button.
Roles are worth naming explicitly. One primary that carries recognition and appears most often, one or two secondary colours that support it, a neutral ramp of greys and off-whites doing most of the actual work on a page, and a small set of functional colours for success, warning and error states. Functional colours are the ones people forget to define, so they get invented at the moment of need, which is how a brand ends up with three different reds.
Contrast belongs in the palette, not in a later accessibility review. Brand colours are frequently chosen as flat swatches and then discovered to be unreadable as text on the brand background. The fix is not to abandon the colour but to decide the pairings once: for each background in the palette, which text colour is approved, and which combinations are permitted for large display type only. Recording approved pairs removes the most common source of unreadable output, and removes it before anybody has to argue about it.
Restraint does more for recognition than range. A palette with twelve colours has no dominant colour, and the dominant colour is the one people remember. It is worth writing down rough proportions alongside the values, because a brand where the primary covers a tenth of the surface and a brand where it covers half are recognisably different brands using identical codes.
Finally, the palette needs a single home. Drift starts with somebody needing a colour, finding the file inconvenient to open, and picking something close. Values held once and read directly by the tools that produce the work are worth more than a beautifully typeset page describing what the colours are meant to evoke.