main.cpp

#include <iostream>

const int RGB_HEX_LENGTH = 7;

int main(){
    std::string input;
    bool alnum;

    do{
        std::cout << "Enter a color in hex format (#RRGGBB):";
        std::getline(std::cin, input);

        if((input.size() != RGB_HEX_LENGTH) || (input[0] != '#')){
            std::cout << "Please enter the color in hexadecimal format, starting with # followed by six hex values\n";
        }
	int x = 1;
	alnum = true;
	while(x < input.size()){
		if (!(isalnum(input[x]))){
			alnum = false;
			std::cout << "Please enter the color in hexadecimal format, starting with # followed by six hex values\n";
			break;
		}
		x++;
	}
    }while( (input.size() != RGB_HEX_LENGTH) || (input[0] != '#') || (alnum == false));

    std::cout << "Your hex color is: " << input << std::endl;

    return 0;
}