But fans of Simpson's trademark blond locks shouldn't despair.
"It's a wig!" Simpson told "Entertainment Tonight" at the awards show, which honors outstanding Latino artistic achievement.
"I put eye shadow on the blond to hide it."
The hairpiece will go on sale as part of Simpson's new line of wigs.
Longoria, who hosted the gala, had recruited the sultry singer to appear in a bit with comedian George Lopez.
In January, at the People's Choice Awards, Lopez joked that Simpson's estranged hubby, Nick Lachey, would be working the show as a seat filler.
Longoria decided to bring Simpson and Lopez together to settle any bad blood.
The ALMA show will be broadcast June 5 on ABC.