brindled cow, bright, excel
犖 centers on a brindled cow, a specific type of spotted or streaked cattle. The meanings 'bright' and 'excel' are rare and likely extended from the striking appearance of such an animal, but the connection is not obvious from the given glosses alone.
犖 combines 牛 (cow) with a phonetic element that may be related to 勞 or a similar sound component. The character originally referred to a brindled cow, and the extended meanings 'bright' and 'excel' are likely derived from the distinctive appearance of such cattle.
The bottom is 牛 (cow), and the top looks like a fire under a cover. Imagine a cow with fiery, streaked patches on its hide—a brindled cow that stands out brightly.
For ラク, picture a brindled cow resting on a rock (ラク sounds like 'rock'): the cow's bright patches shine as it lies on the rock.