Function hexColorValue

hexColorValue(val: string): string
Validates and normalizes a hexadecimal color value.

Accepts either "auto" or a 6-character RGB hex value (with or without # prefix). The # prefix is commonly used but technically invalid in OOXML, so it is stripped for strict compliance.

Reference: ST_HexColor in OOXML specification
Parameters
- val: string
  The color value to validate ("auto" or hex color)
Returns string
The normalized color value
Throws
Error if the hex color is invalid
Example
```
const color1 = hexColorValue("auto"); // Returns "auto"
const color2 = hexColorValue("FF0000"); // Returns "FF0000"
const color3 = hexColorValue("#00FF00"); // Returns "00FF00" (# stripped)
```
- Defined in util/values.ts:267