The mention of coffee grains makes me speculate if I feed tea leaves to chickens, will they lay pre-tea-stained eggs? Inquiring minds want to know
Chickens are indeed highly omnivorous, they will even kill mice and other vermin if they can get hold of them and then eat them, let alone small insects. This appears to horrify some amateur chicken-keepers, especially when a cat flushes mice and the chickens go for the mice. Chickens are about as harmless and docile as, well, a cat.
(The yolk colour varying with diet is also well known to me, though perhaps I know more domestic chicken keepers than average).
If you like your eggs very yellow, you can always add a little turmeric and cumin to the scrambled eggs, which tastes great.