The 19-year-old was named in the Dragons' pre-tournament training camp on Monday and has joined the likes of Aaron Ramsey and Gareth Bale in Portugal after making only six first-team appearances for Cardiff.
Colwill was playing academy football at the start of the season, but now has the opportunity to force his way into the 26-man squad that Wales boss Robert Page will announce on Sunday.
"It's been unbelievable," Colwill said from Wales' training base on the Algarve.
"To train with the players I've watched since I was a kid and cheered on in Euro 2016.
"The standard's just unbelievable, I'm definitely pinching myself. The manager has just said train as well as you can and see where it goes from there.
"It will be really tough (being selected for Euro 2020) because there's loads of good players here, but you've got to reach for the stars so I guess it is possible.
"It's what you dream of doing when you're a kid, playing for your country. It would be great if it comes true, but if not then hopefully in the years to come."
Colwill was educated at the same Neath school as Tottenham defender Ben Davies, who began his career at nearby Swansea.
But Colwill made the longer journey up the M4 at the age of eight to join Cardiff's academy, progressing through the various age groups before stepping up to the under-23 side ahead of the 2020-21 campaign.
There was no hint of Colwill's amazing journey when he began the season sitting on the bench for Cardiff Under-23s in a 5-1 defeat at Swansea.
But he was handed a first-team chance by new Bluebirds boss Mick McCarthy in February and made his Wales Under-21s debut against the Republic of Ireland the following month.
Colwill started Cardiff's final three Sky Bet Championship games of the season and was called into Wales' EFL training camp before making the cut for Portugal.
"He has been great," Colwill said of McCarthy's role in his rapid rise. "He is positive and encourages me.
"He knows my qualities and told me to show them here. I just take it in my stride and do as well as I can every day.
"I came into the EFL camp not knowing I could be picked for this, and it was a bit surreal when the gaffer (Page) told me I was coming here.
"Each level gets a bit quicker. You get used to a level, you think it's OK and then you go up to the next one and it's even quicker.
"You've got to adjust to how you play and how many touches you take.
"It's been a crazy year. It has come so fast and football changes so quick. Hopefully I will keep going on and get picked for the next one."