Fix set color for gradient

[heyuncle]

Overview: What does this pull request change?

• Fixes "The issue of inaccurate coloring with set_color(). The issue of inaccurate coloring with set_color(). #4198"

Motivation and Explanation: Why and how do your changes improve the library?

Links to added or changed documentation pages

Further Information and Comments

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested