Better label color validation, fixes #7454

##### What does this MR do?

This PR improves the label color validation. Color isn't allowed to be empty. Additional it adds testcases for invalid input.

Why was this MR needed?

500 error is raised if color is empty.

What are the relevant issue numbers?

This PR fixes #7454

Prefer single-quoted strings when you don't need string interpolation or special symbols.