Thousands of people turned out to watch the Queen present The Royal Welsh regiment with new colours.
The honour formally recognises The Royal Welsh, which was formed in 2006 following the merger of three Welsh regiments.
About 600 soldiers marched from Cardiff Castle to the Millennium Stadium for the ceremony on Thursday, watched by about 10,000 people.
Crowds gathered at Cardiff Central station to see the Queen arrive.
Ronald Auston, who served with the 2nd Battalion in the 1950s, said the event was "spectacular".
Kevin Johnson, who served with the South Wales Borderers which amalgamated with the Royal Welsh regiment, said the event "brings back memories".
"It was emotional, a good day of entertainment. It's a proud moment," he said.
After the parade, the Queen met soldiers and their families.
Brig Philip Napier said: "All those soldiers on parade will understand the history of the regiment and what it means to be a Royal Welshman."
Colours are flags originally carried into battles the regiment has been involved with in the past.
About 30 teenage students from Cardiff's Military Preparation College acted as stewards, marshalling the VIPs at the event.
Huw Lewis, founder and managing director of the college, said: "Being part of this event hosted by Her Majesty the Queen is a real honour and privilege for our students.
"The overall experience, which is a once in a lifetime opportunity, will no doubt inspire them further to pursue a rewarding career in the military."