A cover letter is primarily a business document that should present you as a professional. Therefore, an excessive riot of colors can distract the attention from the text. However, creative design is appropriate if:
It is placed on one part of the page and does not affect the text itself. In this case, your document is still easy to read and understand.
The layout contains the color scheme of the company to which you are applying. Such coincidence can bring you extra points. To achieve it, browse the company's logo, products, and website.
You apply for a position where your originality and aesthetic sense matter. The creative cover letter template encourages the recruiter to jump to your portfolio.