MINOR: input validation for color converter

[Segelzwerg]

After the merge of #41 the color converter requires checks for valid inputs of HEX color strings.

[HealthyDoggo]

How do you want the error to be thrown if an input passed is not a valid HEX color string?

[Segelzwerg]

How do you want the error to be thrown if an input passed is not a valid HEX color string?

Hi @HealthyDoggo thanks for contributing.

How about we create a custom error that is called something NonHexValueError?

[HealthyDoggo]

Sounds good.