basically flutter uses color AARRGGBB format you can use below color code with any color property like:
new Container(color: const Color(0xff2980b9));
AA = transparency
RR = red
GG = green
BB = blue
now if you want to create custom color 8-digit code from 6-digit color code then just append transparency (AA) value to it
Transparency percentages
Following are some example transparency percentages and their hex values
100% - FF
95% - F2
90% - E6
85% - D9
80% - CC
75% - BF
70% - B3
65% - A6
60% - 99
55% - 8C
50% - 80
45% - 73
40% - 66
35% - 59
30% - 4D
25% - 40
20% - 33
15% - 26
10% - 1A
5% - 0D
0% - 00
in my case i always use AA = ff because 6-digit color has ff transparency.
for 6-digit color best site