Coloring cube

input/code.cpp: In function 'char get_next_color(int*)':
input/code.cpp:9:14: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
    9 |     if (*lui == colors.size()) {
      |         ~~~~~^~~~~~~~~~~~~~~~

#include <iostream>
#include <vector>

std::vector<char> colors = {'R', 'G', 'B'};

char get_next_color(int *lui) {
    (*lui)++;

    if (*lui == colors.size()) {
        *lui = 0;
    }

    return colors.at(*lui);
}

int main(int argc, char **argv) {
    int n;
    std::cin >> n;

    int lui = -1;

    for (int i=0; i<n; i++) {
        for (int j=0; j<n; j++) {
            for (int k=0; k<n; k++) {
                std::cout << get_next_color(&lui);
                if (k != n-1) std::cout << " ";
            }
            std::cout << std::endl;
        }
        if (i != n-1) std::cout << std::endl;
    }

    return 0;
}

Test 1

Verdict: WRONG ANSWER

Test 2

Verdict: ACCEPTED

Test 3

Verdict: WRONG ANSWER

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: WRONG ANSWER

Test 8

Verdict: ACCEPTED

Test 9

Verdict: WRONG ANSWER

Test 10

Verdict: ACCEPTED