def create_marker_styles(num_vars):
    """Create pairs of colors and shapes for covariance plot markers.

    Sufficient for 4*7=28 variables.
    """
    markers = ['o', 's', '^', '*']
    colors = ['b', 'g', 'r', 'c', 'm', 'y', 'k']
    styles = []

    for marker in markers:
        for color in colors:
            styles.append((color, marker))

    all_colors = []
    all_markers = []

    for style in styles:
        all_colors.append(style[0])
        all_markers.append(style[1])

    out_colors = all_colors[:num_vars]
    out_markers = all_markers[:num_vars]

    pairs = [out_colors, out_markers]

    return pairs