Color symbolism is the use of color as a representation or meaning of something that is usually specific to a particular culture or society. Context, culture and time are certainly important factors to consider when thinking about color symbolism.
Definition of backpack for English Language Learners. : a bag for carrying things that has two shoulder straps and is carried on the back.
Backpacks are measured by the total capacity of all of the pockets in litres, essentially you will be able to fit in 30 litres of gear in a 30 litre backpack. This however might not mean a great deal when you are trying to decide if a rucksack is big or small enough for your needs.