Cards changing mana color
There are nine cards that allow you to spend mana as though it were another color:


Mycosynth Lattice lets you spend mana as though it were mana of any color without restriction.
Daxos of Meletis and Psychic Intrusion allow you to cast a spell from an opponent's library, and allow you to spend mana as though it were mana of any color when paying that spell's cost.
Celestial Dawn and False Dawn both let you tap your lands for white mana and spend white mana as though it were mana of any color. False Dawn additionally makes any other mana you generate white.
North Star lets you spend mana as though it were mana of any color for a single spell, and Unexpected Potential lets you spend mana as though it were mana of any color for spells with a chosen name.
Quicksilver Elemental lets you spend blue mana as though it were mana of any color for its own abilities (which it copies from other creatures).
Sunglasses of Urza lets you spend white mana as though it were red.


There are also 58 cards that allow you to spend mana to get colored mana. I'm not going to list all of them, but here are some highlights:


Bog Initiate filters an arbitrary amount of mana into black mana.
Farrelite Priest filters an arbitrary amount of mana into white mana, with the caveat that you have to sacrifice it if you activate the ability 4 or more times in one turn. Initiates of the Ebon Hand does the same for black mana.
Orochi Leafcaller filters an arbitrary amount of green mana into mana of any color. Nomadic Elf does the same, but at an additional cost of one mana per mana generated.
A cycle of cards like An-Havva Township, which let you tap and pay one mana for one color, or tap and pay two mana for your choice of two other colors.
A cycle of lands from Eventide like Cascade Bluffs, which let you tap and pay one mana in your choice of two colors to get two mana in any combination of those colors.
A cycle of lands like Darkwater Catacombs, which let you tap and pay one mana to get one mana in each of two colors.
The Ravnica guild signets, like Boros Signet, which let you tap and pay one mana to get one mana in each of the guild's two colors.
The land Crystal Quarry, which lets you tap and pay 5 mana to get one mana of each color.