Thinking a bit more about rarities... the word "rarity" is mostly wrong in the Terraria context. The colour is not an indication of
rarity, per se, it's an indication of
tier. The Red and Purple 'rarity' things are relatively easy to obtain - most are craftable from guaranteed drops. Compared to an example rare item, something difficult to obtain: The Rod of Discord is Lime. So is the Ankh Shield. Both are arguably rarer than Solar Flare Armour.
Coins, materials, most furniture and so on are all White rarity because they're available at the very start of the game (though are not necessarily easy to obtain).
Perhaps the developers and/or the Wiki should abandon the 'rarity' descriptor in favour of a 'tier' descriptor.
Since Pumps are constructed with Wire and Wire is sold by the Mechanic after Skeletron is defeated, they could be Green or Orange rarity. A lot of what the Mechanic sells is Blue rarity, so I'd be fine with making Pumps Blue rarity as well. That answers my own question from before.
Blue rarity it is!