# POST Validate adding a letter part's demo colours

Dry-runs the demo colours operation without persisting anything: the same checks and the same problems it would return, or a 200 verdict when it would succeed. Nothing is written, no colour selection is created, no caches are touched, no idempotency key is consumed, and no option-write budget is spent. The verdict is advisory. Requires `customisers:write`; the request takes no body.

- Source URL: https://www.signcustomiser.com/help/api/v3-post-validate-adding-a-letter-part-s-demo-colours/
- Markdown URL: https://www.signcustomiser.com/help/api/v3-post-validate-adding-a-letter-part-s-demo-colours.md
- Group: Customiser letter parts
- Method: POST
- Path: /api/v3/customisers/{customiser_id}/letter-types/{letter_type_id}/letter-parts/{letter_part}/colours/validate-populate-demo
- Auth: Bearer token
- Required scopes: `customisers:write`

## Path parameters

- customiser_id (string, required): The customiser id. Example: 1
- letter_type_id (string, required): The letter type id. Example: 7
- letter_part (string, required): The letter part. Example: face

## cURL example

```bash
curl https://web.signcustomiser.com/api/v3/customisers/1/letter-types/7/letter-parts/face/colours/validate-populate-demo \
  --request POST \
  --header 'Accept: application/json' \
  --header 'Authorization: Bearer YOUR_API_TOKEN'
```

## Response examples

### 200 POST 200 example 1

```json
{
  "data": {
    "object": "letter_part_colour_validation",
    "operation": "populate_demo",
    "customiser_id": 42,
    "letter_type_id": 7,
    "letter_part": "face",
    "colour_id": null,
    "valid": true
  },
  "links": {
    "self": "https://web.signcustomiser.com/api/v3/customisers/42/letter-types/7/letter-parts/face/colours/validate-populate-demo",
    "documentation": "https://www.signcustomiser.com/help/api/"
  },
  "meta": {
    "api_version": "v3",
    "request_id": "req_01jz9x2k7c8f3m5n6p7q8r9s0t"
  }
}
```

### 401 Missing API key

```json
{
  "type": "https://www.signcustomiser.com/help/api/problems/missing_api_key",
  "title": "Missing API key",
  "status": 401,
  "code": "missing_api_key",
  "detail": "Provide a store API key as a bearer token.",
  "request_id": "req_01jz9x2k7c8f3m5n6p7q8r9s0t"
}
```

### 403 Insufficient scope

```json
{
  "type": "https://www.signcustomiser.com/help/api/problems/insufficient_scope",
  "title": "Insufficient scope",
  "status": 403,
  "code": "insufficient_scope",
  "detail": "This operation requires the customisers:write scope.",
  "required_scopes": [
    "customisers:write"
  ],
  "granted_scopes": [
    "customisers:read"
  ],
  "request_id": "req_01jz9x2k7c8f3m5n6p7q8r9s0t"
}
```

### 404 Unknown letter type

```json
{
  "type": "https://www.signcustomiser.com/help/api/problems/resource_not_found",
  "title": "Resource not found",
  "status": 404,
  "code": "resource_not_found",
  "detail": "The requested resource does not exist or does not belong to the authenticated store.",
  "request_id": "req_01jz9x2k7c8f3m5n6p7q8r9s0t"
}
```

### 409 Letter part already has colours

```json
{
  "type": "https://www.signcustomiser.com/help/api/problems/letter_part_colours_not_empty",
  "title": "Letter part colours not empty",
  "status": 409,
  "code": "letter_part_colours_not_empty",
  "detail": "The face letter part already holds colours, so demo colours would mix into them. Delete its colours first, or add colours one at a time.",
  "errors": [
    {
      "pointer": "",
      "code": "letter_part_colours_not_empty",
      "detail": "The face letter part already holds colours."
    }
  ],
  "request_id": "req_01jz9x2k7c8f3m5n6p7q8r9s0t"
}
```

### 422 No demo colours for this part

```json
{
  "type": "https://www.signcustomiser.com/help/api/problems/demo_colours_not_available",
  "title": "Demo colours not available",
  "status": 422,
  "code": "demo_colours_not_available",
  "detail": "Sign Customiser has no demo colours for the side letter part of this sign category. Add colours one at a time instead.",
  "errors": [
    {
      "pointer": "",
      "code": "demo_colours_not_available",
      "detail": "No demo colours exist for this sign category and letter part."
    }
  ],
  "request_id": "req_01jz9x2k7c8f3m5n6p7q8r9s0t"
}
```

### 429 Rate limited

```json
{
  "type": "https://www.signcustomiser.com/help/api/problems/rate_limited",
  "title": "Too many requests",
  "status": 429,
  "code": "rate_limited",
  "detail": "You have exceeded the request limit for this API key.",
  "retry_after": 60,
  "request_id": "req_01jz9x2k7c8f3m5n6p7q8r9s0t"
}
```