The HSB (Hue, Saturation, Brightness) model, popularised by digital design tools, breaks colour into three axes: the colour itself (hue), how pure it is (saturation), and how much light it appears to reflect (brightness). Of the three, brightness has the most direct impact on accessibility — it is the axis that determines whether text is readable against a background.
A useful exercise is to convert a design to greyscale. What remains readable and what collapses tells you whether your colour decisions are sound. If hierarchy, contrast, and meaning survive the greyscale conversion, the design has a solid luminance foundation. Colour can then do emotional and semantic work on top of that solid base.
In dark-mode design, brightness is the primary tool for creating surface hierarchy. Where light-mode designs use light colours to create depth, dark-mode designs use slight brightness increases above a dark base. The surface-container system used in Material Design and many design tokens codifies this: each tier of the stack is one step brighter than the one beneath it.