[Go] make `color` optional in `observe`

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (0f6ba86) 88.12% compared to head (047edfa) 88.10%.

Files	Patch %	Lines
pgx/_src/games/go.py	75.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1138 +/- ## ========================================== - Coverage 88.12% 88.10% -0.02% ========================================== Files 61 61 Lines 5935 5937 +2 ========================================== + Hits 5230 5231 +1 - Misses 705 706 +1 ```

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