Add kwarg for default color similarity threshold in gui()

[tmcgrath325]

Adds a default_threshold = 0.2 to gui() to more easily allow for the default color similarity threshold value to be tailored to a user's needs.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.29%. Comparing base (07f7c42) to head (81147bf).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #13   +/-   ##
=======================================
  Coverage   98.29%   98.29%           
=======================================
  Files           4        4           
  Lines         293      293           
=======================================
  Hits          288      288           
  Misses          5        5           

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