[Northumberland] Update pin colours.

[dracos]

For FD-6634. [skip changelog]

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.77%. Comparing base (af2a1c4) to head (3269641).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
perllib/FixMyStreet/Cobrand/Northumberland.pm	0.00%	1 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5852      +/-   ##
==========================================
+ Coverage   82.76%   82.77%   +0.01%     
==========================================
  Files         458      458              
  Lines       35906    35933      +27     
  Branches     5884     5896      +12     
==========================================
+ Hits        29719    29745      +26     
  Misses       4472     4472              
- Partials     1715     1716       +1     

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