while (col < 'a' || col > 'h') {
    printf("Input a column (a...h)\n");
    scanf("%c", &col);
}

Input a column (a...h)
y
Input a column (a...h)
Input a column (a...h)
z
Input a column (a...h)
Input a column (a...h)

while (lin < 1 || lin > 8) {
    printf("\nInput a line (1...8)\n");
    scanf("%d", &lin);
}

#include <stdio.h>

int get_color(int l, int c);

int main() {
    char col = '\0';
    int lin = 0;

    while (col < 'a' || col > 'h') {
        printf("Input a column (a...h)\n");
        fflush(stdin);
        scanf(" %c", &col);
    }

    while (lin < 1 || lin > 8) {
        printf("\nInput a line (1...8)\n");
        fflush(stdin);
        scanf("%d", &lin);
    }
    printf("This is a %s square.", (get_color(lin, col)) ? "black" : "white");
        return 0;
}

//Returns 0 for white, 1 for black.