Validate color format of category in BE

[ivarshav]

No description provided.

[yammesicka]

Awesome job! Again, added some minor comments :)

[codecov-io]

Codecov Report

Merging #251 (9bf6f63) into develop (5d0a09d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #251   +/-   ##
========================================
  Coverage    99.40%   99.40%           
========================================
  Files           47       47           
  Lines         2188     2199   +11     
========================================
+ Hits          2175     2186   +11     
  Misses          13       13           

Impacted Files	Coverage Δ
app/routers/categories.py	100.00% <100.00%> (ø)

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