castomAPI
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POST /v1/convert/color free No key needed Convert

Convert colors across HEX

Convert colors across HEX, RGB, HSL, HSV, CMYK, and CIE LAB spaces, and compute WCAG 2.1 contrast ratios and accessibility metrics

POST

Request

  • x-api-key: <your-api-key>
  • Content-Type: application/json

Response 

{
  "status": "success",
  "colorSpaces": {
    "hex": "#7C4DFF",
    "hex8": "#7C4DFFFF",
    "rgb": {
      "r": 124,
      "g": 77,
      "b": 255,
      "a": 1,
      "css": "rgb(124, 77, 255)"
    },
    "hsl": {
      "h": 256,
      "s": 100,
      "l": 65,
      "a": 1,
      "css": "hsl(256, 100%, 65%)"
    },
    "hsv": {
      "h": 256,
      "s": 70,
      "v": 100,
      "a": 1,
      "css": "hsv(256, 70%, 100%)"
    },
    "cmyk": {
      "c": 51,
      "m": 70,
      "y": 0,
      "k": 0,
      "css": "cmyk(51%, 70%, 0%, 0%)"
    },
    "lab": {
      "l": 47.3,
      "a": 51.8,
      "b": -80.6,
      "css": "lab(47.3% 51.8 -80.6)"
    }
  },
  "luminance": 0.1621,
  "isDark": true,
  "accessibility": {
    "contrastRatio": 4.62,
    "wcagAA": true,
    "wcagAAA": false,
    "wcagAALarge": true,
    "comparisonColor": "#FFFFFF"
  },
  "tokensDeducted": 1
}
The response below is a mock built from the endpoint's own schema — no request leaves this page. Edit the body, press Send, read the shape you will get back. Run it for real in the dashboard →

What it does

Give it a colour in any CSS notation — hex, rgb(), hsl(), a named colour, or an object — and it answers the same colour in six spaces plus the relative luminance. Add a comparisonColor and it computes the WCAG 2.1 contrast ratio and the AA / AAA verdicts for normal and large text, which is how this site's own palette was checked.

When to use it

A theme editor that must refuse an unreadable combination; a design-token pipeline; generating a print-ready CMYK from a brand hex.

Request

Body

FieldTypeRequiredDescription
colorstring | objectyesColor string (e.g. "#FF5733", "rgb(255, 87, 51)", "hsl(11, 100%, 60%)") or object ({ r: 255, g: 87, b: 51 }).
comparisonColorstring | objectnoOptional secondary color to evaluate relative contrast ratio and custom WCAG accessibility compliance.

Example request

{
  "color": "#7C4DFF",
  "comparisonColor": "#FFFFFF"
}

Response

FieldTypeDescription
accessibilityobject
  ↳ againstBlackobject
  ↳ againstCustomobject
  ↳ againstWhiteobject
colorSpacesobject
  ↳ cmykobject
  ↳ hexstring
  ↳ hex8string
  ↳ hslobject
  ↳ hsvobject
  ↳ labobject
  ↳ rgbobject
isDarkboolean
luminancenumber
statusstring
tokensDeductedinteger

Example response

{
  "status": "success",
  "colorSpaces": {
    "hex": "#7C4DFF",
    "hex8": "#7C4DFFFF",
    "rgb": {
      "r": 124,
      "g": 77,
      "b": 255,
      "a": 1,
      "css": "rgb(124, 77, 255)"
    },
    "hsl": {
      "h": 256,
      "s": 100,
      "l": 65,
      "a": 1,
      "css": "hsl(256, 100%, 65%)"
    },
    "hsv": {
      "h": 256,
      "s": 70,
      "v": 100,
      "a": 1,
      "css": "hsv(256, 70%, 100%)"
    },
    "cmyk": {
      "c": 51,
      "m": 70,
      "y": 0,
      "k": 0,
      "css": "cmyk(51%, 70%, 0%, 0%)"
    },
    "lab": {
      "l": 47.3,
      "a": 51.8,
      "b": -80.6,
      "css": "lab(47.3% 51.8 -80.6)"
    }
  },
  "luminance": 0.1621,
  "isDark": true,
  "accessibility": {
    "contrastRatio": 4.62,
    "wcagAA": true,
    "wcagAAA": false,
    "wcagAALarge": true,
    "comparisonColor": "#FFFFFF"
  },
  "tokensDeducted": 1
}

Errors

Every error answers with the same envelope: an HTTP status, a machine code and a sentence.

400The body or a parameter failed validation. The details array names the field.
401No key, a revoked key or a wrong header. Send x-api-key.
402The wallet cannot cover this call. Top up or wait for the monthly grant.
404The resource does not exist, or it is not yours. Both answer identically and cost nothing.
429Over the per-minute quota of your tier. Back off and retry.
5xxUpstream refused or timed out. The call is not billed.

Code

curl -X POST 'https://app.castomapi.com/v1/convert/color' \
  -H 'x-api-key: <your-api-key>' \
  -H 'Content-Type: application/json' \
  -d '{"color": "#7C4DFF", "comparisonColor": "#FFFFFF"}'
const res = await fetch('https://app.castomapi.com/v1/convert/color', {
  method: 'POST',
  headers: { 'x-api-key': '<your-api-key>', 'Content-Type': 'application/json' },
  body: JSON.stringify({"color": "#7C4DFF", "comparisonColor": "#FFFFFF"})
});
const data = await res.json();
console.log(data);
import { request } from 'node:https';

// Node 18+ has fetch built in
const res = await fetch('https://app.castomapi.com/v1/convert/color', {
  method: 'POST',
  headers: { 'x-api-key': '<your-api-key>', 'Content-Type': 'application/json' },
  body: JSON.stringify({"color": "#7C4DFF", "comparisonColor": "#FFFFFF"})
});
const data = await res.json();
console.log(data);
import requests

res = requests.post(
    'https://app.castomapi.com/v1/convert/color',
    headers={'x-api-key': '<your-api-key>'},
    json={"color": "#7C4DFF", "comparisonColor": "#FFFFFF"},
    timeout=30,
)
print(res.json())
<?php
$ch = curl_init('https://app.castomapi.com/v1/convert/color');
curl_setopt_array($ch, [
    CURLOPT_CUSTOMREQUEST => 'POST',
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_HTTPHEADER => ['x-api-key: <your-api-key>', 'Content-Type: application/json'],
    CURLOPT_POSTFIELDS => json_encode(['color' => '#7C4DFF', 'comparisonColor' => '#FFFFFF']),
]);
$data = json_decode(curl_exec($ch), true);
print_r($data);

Cost and limits

This operation needs no API key and costs nothing: called without a key it answers 200 and deducts zero tokens, up to 10 requests a minute per IP address.

Rate limits per minute

No key10
Free60
Pro300
Enterprise1200

Docs → Pricing

Questions

What does Convert colors across HEX cost?

This operation needs no API key and costs nothing: called without a key it answers 200 and deducts zero tokens, up to 10 requests a minute per IP address.

How do I authenticate?

Send your key in the x-api-key header. One key covers every metered endpoint; there is nothing to enable per endpoint.

Can I send an Authorization: Bearer token instead?

Not for a metered endpoint — those take the x-api-key header and nothing else. Bearer tokens belong to the account side: the dashboard and account APIs accept a JWT session, and castomapi-auth issues those through a standard OAuth2 handshake, with authorization code, client credentials and refresh grants. The auth service page covers it.

What are the limits?

10 requests a minute per IP with no key at all, 60 a minute on the free tier, 300 on Pro, 1200 on Enterprise. Above the no-key tier the quota is per account, across every endpoint.

OpenAPI: https://app.castomapi.com/docs/json