As of 09/2020, GitHub uses this color palette for labels:
Here are the hex codes:
#b60205
#e99695
#d93f0b
#f9d0c4
#fbca04
#fef2c0
#0e8a16
#c2e0c6
#006b75
#bfdadc
#1d76db
#c5def5
#0052cc
#bfd4f2
#5319e7
#d4c5f9