Returns the list of custom color definitions for the TikZ file.

def _get_color_definitions(data):
    """Returns the list of custom color definitions for the TikZ file.
    """
    definitions = []
    fmt = "\\definecolor{{{}}}{{rgb}}{{" + ",".join(3 * [data["float format"]]) + "}}"
    for name, rgb in data["custom colors"].items():
        definitions.append(fmt.format(name, rgb[0], rgb[1], rgb[2]))
    return definitions