Add color selection for stroke order

[cyyeong]

Tested gen.py on local.

[codecov-commenter]

Codecov Report

Merging #50 into master will decrease coverage by 0.15%.
The diff coverage is 23.07%.

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
- Coverage   34.40%   34.24%   -0.16%     
==========================================
  Files           9        9              
  Lines         872      876       +4     
==========================================
  Hits          300      300              
- Misses        572      576       +4     

Impacted Files	Coverage Δ
backend/src/server.py	0.00% <0.00%> (ø)
backend/src/gen.py	34.62% <27.27%> (-0.23%)	⬇️

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