All you need to do is convert the string to integers and then split them into three separate r, g, b values.
string hexstring = "#FF3Fa0";
// Get rid of '#' and convert it to integer
int number = (int) strtol( &hexstring[1], NULL, 16);
// Split them up into r, g, b values
int r = number >> 16;
int g = number >> 8 & 0xFF;
int b = number & 0xFF;
You may want to have a look at this question as well.
Edit (thanks to James comments):
For some machine (e.g. Arduino (Uno)), ints are 16 bits instead of 32. If red values are dropping for you, use a long instead.
string hexstring = "#FF3Fa0";
// Get rid of '#' and convert it to integer
long number = strtol( &hexstring[1], NULL, 16);
// Split them up into r, g, b values
long r = number >> 16;
long g = number >> 8 & 0xFF;
long b = number & 0xFF;
Edit (an even safer version, use strtoll
instead of strtol
):
long long number = strtoll( &hexstring[1], NULL, 16);
// Split them up into r, g, b values
long long r = number >> 16;
long long g = number >> 8 & 0xFF;
long long b = number & 0xFF;