Function isValidHexColor

isValidHexColor(hexString: string): boolean
Validates whether a string is a valid "simple color" as defined by the WHATWG HTML specification.

A valid simple color is exactly 7 characters: a # followed by 6 hexadecimal digits (0-9, a-f, A-F). This is the format required for HTML color input elements and many CSS properties.

Note: This function does NOT accept 3-digit shorthand hex codes (e.g., #FFF), as they are not considered "simple colors" per the WHATWG specification.
Parameters
- hexString: string
  The string to validate as a hex color.
Returns boolean
true if the string is a valid 6-digit hex color with leading #, false otherwise.
Example
```
// Valid simple colors
isValidHexColor('#FFFFFF'); // => true
isValidHexColor('#123456'); // => true
isValidHexColor('#abcdef'); // => true
isValidHexColor('#AbCdEf'); // => true
```
Example
```
// Invalid: 3-digit shorthand
isValidHexColor('#FFF'); // => false
isValidHexColor('#000'); // => false
```
Example
```
// Invalid: missing # prefix
isValidHexColor('FFFFFF'); // => false
```
Example
```
// Invalid: wrong characters
isValidHexColor('#xyz123'); // => false
isValidHexColor('#GGGGGG'); // => false
```
Example
```
// Invalid: wrong length
isValidHexColor('#12345');  // => false
isValidHexColor('#1234567'); // => false
```
See
HTML Spec: Valid Simple Color
- Defined in color/isValidHexColor.ts:41