When you request a Google Calendar event it will come with a colorId which is either undefined if user didn't select a colour, or between one and 11 if they did. Since I needed these for Agander, I decided to collate these for the curious. These are the corresponding colours used in the Google Calendar app.
Color ID
Color Name
Hex Code
undefined
Who knows
#039be5
1
Lavender
#7986cb
2
Sage
#33b679
3
Grape
#8e24aa
4
Flamingo
#e67c73
5
Banana
#f6c026
6
Tangerine
#f5511d
7
Peacock
#039be5
8
Graphite
#616161
9
Blueberry
#3f51b5
10
Basil
#0b8043
11
Tomato
#d60000