n_colors: constrain colors between 0 and 255

[AaronStiff]

Code PR

• I have read through the contributing notes and understand the structure of the package. In particular, if my PR modifies code of plotly.graph_objects, my modifications concern the codegen files and not generated files.
• I have added tests (if submitting a new feature or correcting a bug) or
  modified existing tests.
• For a new feature, I have added documentation examples in an existing or
  new tutorial notebook (please see the doc checklist as well).
• I have added a CHANGELOG entry if fixing/changing/adding anything substantial.
• For a new feature or a change in behaviour, I have updated the relevant docstrings in the code to describe the feature or behaviour (please see the doc checklist as well).

Fixes #4105

[alexcjohnson]

@AaronStiff This looks great! Let's just add two things to the PR: a test, perhaps in test_colors.py, and a changelog entry (after merging in the latest master)

[alexcjohnson]

💃 Thanks!