/** An sRGB colour, 0-255 per channel. */
export interface Rgb {
    r: number;
    g: number;
    b: number;
}
/** Parses `#abc` or `#aabbcc` (with or without the hash). Returns null for anything else. */
export declare function parseHex(colour: string): Rgb | null;
/** `{r,g,b}` back to a `#rrggbb` string. */
export declare function toHex({ r, g, b }: Rgb): string;
/**
 * Straight sRGB interpolation between two hex colours, `t` clamped to 0..1.
 *
 * sRGB rather than a perceptual space is deliberate: both endpoints are colours an operator
 * picked in a colour input, so the midpoint should be the one they would predict from mixing
 * them. Falls back to whichever endpoint parses when the other is malformed.
 */
export declare function lerpHex(from: string, to: string, t: number): string;
/** WCAG relative luminance, 0 (black) to 1 (white). Returns null for an unparseable colour. */
export declare function relativeLuminance(colour: string): number | null;
/**
 * Black or white, whichever reads better on the given background.
 *
 * Needed wherever a value ramp is used as a fill behind text: a ramp runs from dark to light by
 * design, so no single fixed text colour stays legible across it.
 */
export declare function readableTextColour(background: string): string;
/** A hex colour as `rgba(...)` at the given alpha, for glows and washes. */
export declare function withAlpha(colour: string, alpha: number): string;
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