ALTER TABLE events
    ADD COLUMN day_title_color CHAR(7) NOT NULL DEFAULT '#ffffff' AFTER text_color;

UPDATE events
SET day_title_color = CASE
    WHEN (
        0.2126 * CONV(SUBSTRING(background_color, 2, 2), 16, 10) +
        0.7152 * CONV(SUBSTRING(background_color, 4, 2), 16, 10) +
        0.0722 * CONV(SUBSTRING(background_color, 6, 2), 16, 10)
    ) / 255 > 0.56 THEN '#0e0e0e'
    ELSE '#ffffff'
END;