Dropped were pitchers Felipe Paulino and Noel Arguelles and infielder Irving Falu. The moves left the roster at an even 40.
Paulino opted to become a free agent rather than accept an outright assignment to Triple-A Omaha. A right-hander, Paulino didn't pitch for Kansas City last season because of elbow surgery and then shoulder surgery. Arguelles and Falu were designated for assignment.
Promotion to the Major League roster protects Colon, Cuthbert, Adams and Mariot from possible selection by another team in the Rule 5 Draft on Dec. 12 at the Winter Meetings.
Unless the Royals acquire another second baseman, Colon, 24, could be a contender for that job along with Emilio Bonifacio, Chris Getz and Johnny Giavotella. Colon was the Omaha Player of the Year with a .273 average, 12 home runs, 58 RBIs and 15 stolen bases in 131 games. He finished strong, hitting .335 in the second half.
MLB.com ranks Colon 17th among the Royals' Top 20 Prospects.
"Christian had a very good second half, he's trending the right way and any time you're on the 40-man roster, especially at this stage of his career, he's got an opportunity to compete," said Royals general manager Dayton Moore.
Cuthbert, 21, is No. 9 on the Royals' Top Prospects list. A right-handed-hitting third baseman from Nicaragua, he divided the season between Class A Wilmington and Double-A Northwest Arkansas. Combined, he batted .247 with 59 RBIs, eight homers and 37 doubles in 124 games.
"We believe in him and believe in his bat long term," Moore said. "He's a very good defender at third base."
Adams, 24, was Wilmington's Player of the Year (.276 average, 39 RBIs in 87 games) but also earned promotions to Northwest Arkansas and Omaha before the season was over. A right-handed hitter, he joined Omaha in the postseason and went 3-for-5 with four RBIs as the Storm Chasers beat Salt Lake for the Pacific Coast League title.
"He's probably the most improved player in our organization," Moore said. "He's got tremendous tools, and a guy who was primarily a basketball player out of high school, and we knew it was going to take a little time and he's progressed well."
Mariot, 25, a right-handed reliever, had a 3.56 ERA, 11 saves and a 4-5 record in 47 outings for Omaha. He also racked up 66 strikeouts against 25 walks in 60 2/3 innings.
"He had a great Spring Training for us and had a tremendous curveball and a very successful season in Triple-A. He was used in a lot of high-leverage situations and he did great," Moore said.
Players first signed at age 18 must be added to 40-man rosters within five seasons or they become eligible to be drafted by other organizations through the Rule 5 process. Players signed at 19 years or older have to be protected within four seasons. Clubs pay $50,000 to select a player in the Major League phase of the Rule 5 Draft. If that player doesn't stay on the 25-man roster for the full season, he must be offered back to his former team for $25,000.
Paulino, 30, had Tommy John surgery on July 3, 2012, and his Minor League rehabilitation this year was complicated by back and shoulder ailments. A highlight of his Royals stay came with a combined 12 2/3 scoreless innings and two victories in a pair of starts against the Yankees in May 2012.
"We still had some interest in bringing him back but weren't sure where it'd work," Moore said. "It's unfortunate he couldn't stay healthy."
Arguelles. 23, got a deal worth about $7 million when he signed out of Cuba in 2010 but was plagued by arm problems. This year, the lefty was 1-8 with a 5.93 ERA in 25 games for Northwest Arkansas.
"He's still young and focused and determined," Moore said. "We needed a spot and had to make a decision right now."
If Arguelles clears waivers, he could return to the organization.
Falu, 30, has spent his entire 11-year career in the Royals organization, finally reaching the Majors in 2012 in his 10th pro season. In parts of two seasons with Kansas City, he got into 25 games and hit .337. In the Minors, he logged 1,151 games and was primarily a shortstop but played every infield and outfield position.
"He's a great story," Moore said. "Irving's a great kid and [I'm] really proud of what he accomplished. He perhaps may look for another opportunity, maybe get in the National League where he'd be a better fit. We really appreciate how he handled himself."