Great tits differ visibly in how wide and how solidly black the ventral stripe is. Males generally have broader stripes than females, and within each sex the stripe varies with condition. Experiments that widen or narrow the stripe artificially change how other birds treat the wearer at feeding sites, which is the evidence that the marking carries information rather than simply reflecting age.
The yellow of the breast comes from carotenoid pigments obtained in the diet, chiefly from caterpillars, so brightness is tied to feeding conditions during feather growth. Birds raised in poor caterpillar years are paler. The black is melanin-based and follows a different logic, being less directly limited by food.
What remains contested is how honest the signal really is. Badge-of-status systems are vulnerable to cheating, and the mechanism that keeps a wide stripe reliable, whether social punishment of over-signallers or a physiological cost, has not been settled for this species.