Improve code coverage of src/ui/mod.rs

[changhc]

Part of #700. The following lines should be covered by the new test cases:

Note that the test case num_to_color_code_match is almost meaningless. It's only for boosting test coverage a bit.

Result with tarpaulin:

|| Tested/Total Lines:
|| src/ui/mod.rs: 31/31
55.77% coverage, 681/1221 lines covered

[o2sh]

Thanks again @changhc, this is much appreciated!

I left a small remark