How can I efficiently filter MagicCards.info searches down to cards castable with only certain mana colours?
What you are asking for is effectively equivalent to "the mana cost does not contain {U}, {B}, {R}, {U/B}, {U/R}, or {B/R}" because those are all of the mana symbols that cannot be paid from your mana pool. This can be equivalently expressed by negating the expression that says "the mana cost contains any one of those symbols". So, the resulting query is


  -(mana>=u or mana>=b or mana>=r or mana>={U/B} or mana>={U/R} or mana>={B/R}) o:"enchant land" o:"mana pool"


Note the - at the beginning that negates the bolded expression. As you can see, this does in fact list every card your search found, plus Trace of Abundance. It also incidentally includes colorless cards and cards with Phyrexian and colorless hybrid mana symbols (e.g. {UP} or {2/R}) because it is possible to play those without spending any mana of their colors at all.