Decks should only be considered a certain color if they have both a source and a usage for that color mana
When determining the coloration of a deck, the only colors which should be included are those with both a source (land, dual land, Woodland Mystic, etc.) and a usage (any card which includes that color somewhere in its mana cost) somewhere in the deck.
When a deck has a card with hybrid mana in the cost (such as Daemogoth Titan, which has hybrid green/black mana), the game adds both colors to the deck's mana coloration, but cards like this can easily be used in a deck with only one of the two colors.
Dual lands often have unique properties, such as Wind-Scarred Crag, which grants 1 life when played and can be used to get either red or white mana. This can be beneficial even in an otherwise mono-white deck and isn't present on any single-colored lands.
In both of these cases, when entering the menu to edit the deck, the game will automatically select filters for entirely useless colors of mana.
IMO, cards which can provide any color of mana, such as Ilysian Caryatid, shouldn't be considered a source for every color because people only use them to get more mana, not to get new colors.