How do you specify colors in CSS?

Question

IT Interview Practice · Accepted Answer

CSS offers several color formats. They produce the same colors — the choice is about readability and whether you need transparency.

```css
.a { color: red; }                      /* named keyword (~150 of them) */
.b { color: #ff5733; }                  /* hex: #RRGGBB */
.c { color: #ff5733cc; }                /* hex with alpha: #RRGGBBAA */
.d { color: rgb(255 87 51); }           /* red green blue (0–255) */
.e { color: rgb(255 87 51 / 0.8); }     /* with 80% opacity */
.f { color: hsl(9 100% 60%); }          /* hue saturation lightness */
.g { color: hsl(9 100% 60% / 0.5); }    /* HSL with alpha */
```

## Why HSL is often the most useful

HSL describes color as **H**ue (0–360° on the color wheel), **S**aturation, and **L**ightness — which maps to how humans think about color, making variations intuitive:

```css
:root { --brand-h: 220; }
.primary       { background: hsl(var(--brand-h) 80% 50%); }
.primary-light { background: hsl(var(--brand-h) 80% 70%); } /* just raise lightness */
.primary-dark  { background: hsl(var(--brand-h) 80% 30%); } /* just lower it */
```

Generating a lighter/darker shade is trivial — change one number. Doing that with hex or rgb requires recomputing all channels.

## Alpha / transparency

```css
background: rgb(0 0 0 / 0.5); /* 50% transparent black — for overlays */
```

The `/ alpha` syntax (0 = transparent, 1 = opaque) is how you make semi-transparent colors, useful for overlays and shadows. (Note: `opacity` on an element fades the *whole* element, including children — alpha colors fade just that color.)

## Modern formats

```css
color: oklch(70% 0.15 220); /* perceptually-uniform; better for color systems */
```

## Why it matters

Knowing the formats lets you pick the right tool: hex/rgb for one-off colors, **HSL/OKLCH for design systems** (easy, intuitive shade variations), and the `/ alpha` syntax for transparency.

HSL especially makes building consistent, themeable color palettes far easier.

Why HSL is often the most useful

Alpha / transparency

Modern formats

Why it matters

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