2019-07-12
Diamond Knight's ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.
2019-07-12
If you somehow control a Diamond Knight for which no color was chosen, its last ability will never trigger.
2019-07-12
You 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."