Fix saving GeoTIFF files with gray colors

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.99%. Comparing base (9f44ad8) to head (74c6080). Report is 3 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #113 +/- ## ========================================== + Coverage 93.97% 93.99% +0.01% ========================================== Files 18 18 Lines 1112 1115 +3 ========================================== + Hits 1045 1048 +3 Misses 67 67 ```

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