Bitter is related to Mouth. Mouth is related to Eat. Eat is related to Necessary. Necessary is related to Vital. 
Light is similar to Pale. Pale is the opposite of Dark. 
Bitter <--RelatedTo--> Mouth <--RelatedTo--> Eat <--RelatedTo--> Necessary <--RelatedTo--> Vital 
Light <--SimilarTo--> Pale <--Antonym--> Dark 