With the same interface everywhere, any two colors can be measured against each other. Three questions cover most real work: how far apart, how readable, how equal.
Distance
Color::distance() scores how different two colors look: 0 is identical, small values are barely distinguishable neighbors:
use PhpColor\Color\Color;
$blue = Color::parse('#3b82f6');
$rose = Color::parse('#f43f5e');
Color::distance($blue, $rose);
// 43.16
Use it to dedupe a palette, find the closest named color, or catch two brand colors that sit too close to tell apart.
Contrast
Color::contrast() returns the WCAG 2.x ratio, the number accessibility guidelines are written in:
Color::contrast($blue, Color::white());
// 3.68 -- AA for large text only
Color::contrast($blue, Color::black());
// 5.71 -- AA for body text
The same blue that fails on white passes on black. Checking both directions before shipping a palette is a one-liner.
Equality
equals() compares resolved color values, not syntax:
$blue->equals(Color::parse('rgb(59 130 246)'));
// true -- same color, different notation
Go deeper: Contrast, distance, and color vision covers AA and AAA levels, APCA, and vision simulation.