Voltaic Construct + Ashnod's Transmogrant + Accomplished Alchemist

Voltaic Construct

Ashnod's Transmogrant

Accomplished Alchemist

How it works

1. Cause Accomplished Alchemist to become an artifact using Ashnod's Transmogrant.
2. Activate Accomplished Alchemist by tapping it, adding at least 3.
3. Activate Voltaic Construct by paying 2, untapping Accomplished Alchemist.
4. Repeat from step 2.

To pull off this infinite mana loop, you first need to turn Accomplished Alchemist into an artifact creature using Ashnod's Transmogrant, which sets the stage for Voltaic Construct to interact with it. Once the Alchemist is an artifact, tapping it to produce three or more mana (perhaps after gaining some life) provides more than enough energy to pay for Voltaic Construct's untap ability. By feeding just two of that generated mana back into the Construct, you untap the Alchemist so it can tap again for profit, netting you infinite colored mana and repeating the cycle indefinitely.