Cheapest priced copy (Core Set 2020): $0.35 at Card Kingdom, near mint, their price list as of .
Diamond Knight 
Artifact Creature — Knight · 1/1 · Uncommon · Core Set 2020 · #224
Vigilance (Attacking doesn't cause this creature to tap.)
As this creature enters, choose a color.
Whenever you cast a spell of the chosen color, put a +1/+1 counter on this creature.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
1 printing · $0.35 to $1.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Core Set 2020 | 224 | Uncommon | $0.35 | $1.99 |
3 rulings from Wizards of the Coast
- 12 July 2019You must choose one of the five colors of Magic as Diamond Knight asks you to choose a color. You can’t choose “artifact,” “colorless,” or “chartreuse.”
- 12 July 2019Diamond Knight’s ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.
- 12 July 2019If you somehow control a Diamond Knight for which no color was chosen, its last ability will never trigger.