Ramos, Dragon Engine
Legendary Artifact Creature — Dragon
Flying
Whenever you cast a spell, put a +1/+1 counter on Ramos, Dragon Engine for each of that spell's colors.
Remove five +1/+1 counters from Ramos: Add 
. Activate only once each turn.
Illustrated by
The Brothers' War Retro Artifacts#47MythicEnglishLegality
Rulings
Ramos’s triggered ability counts the number of colors a spell has (from zero to five), not how many colored mana symbols are there in its mana cost or how many colors of mana you spent.
If you cast a colorless spell, Ramos’s triggered ability triggers, but it won’t get any +1/+1 counters.
Ramos’s triggered ability doesn’t trigger until you’ve finished casting a spell, including paying all of its costs. If Ramos has fewer than five +1/+1 counters on it, there’s no way to add counters with a spell and pay for that spell with the newly-added counters.
Ramos’s triggered ability resolves before the spell that caused it to trigger. The ability will resolve even if that spell is countered. If that spell is countered, use its last known information to determine what colors it was.