Since winning American Idol in 2005, country singer Carrie Underwood has become one of the show's highest selling graduates, but she warned the latest crop of Idol finalists to stay true to their fans if they want to find similar success.

Underwood, who on Monday spoke to reporters during a conference call, will star next month in a holiday special, and she just released her second consecutive chart-topping album, Play On.

As it happens, three finalists from this past season of top-rated U.S. television show American Idol, a reality singing competition, are about to release their own albums.

I feel like any contestant that comes off 'Idol' that does well on 'Idol' has to stay true to what people voted for, Underwood said, in response to a question.

I've seen it happen a few times where people are one thing on the show, and then they got off the show and try to do something else, she said.

Underwood, 26, said that she finds it really strange when Idol contestants change their musical style after leaving the show, because through their votes viewers tell contestants what they want.

On Tuesday, Idol champion Kris Allen will release his first album since winning the title in May. Allen told Reuters this month that his self-titled disc will include elements from different musical styles, from pop to the blues.

Also, Idol runner-up Adam Lambert's album For Your Entertainment will hit stores on November 23, and fourth-place finisher Allison Iraheta will release her album Just Like You on December 1.

Underwood said she was a fan of Kris Allen's this year, and also third-place finisher Danny Gokey, whose country-style debut album is expected early next year.

Meanwhile, the four-time Grammy winner will star on December 7 in a two-hour variety program called Carrie Underwood: An All-Star Holiday Special, on Fox, which is also the network behind American Idol.

Underwood will be joined on-stage by country icon Dolly Parton, Grammy winner Brad Paisley, 2008 Idol winner David Cook and by singer and actress Kristin Chenoweth.

Underwood said that when she performed alongside Parton recently, it was a dream come true.

When Dolly's there, you just sort of step out of the spotlight and let her do her thing, because no matter how hard you try she's going to steal the spotlight, and it's great, Underwood said.

Former Idol contestant Jennifer Hudson, who won a Grammy this year for her debut album, will also star in a holiday special this season. Her program, Home for the Holidays, will air on December 14 on ABC.

American Idol