Punters are predicting the Queen will choose blue as the colour of her birthday hat when she celebrates her 90th on Thursday.
Bookmakers Coral are offering odds of 4/1 that the monarch picks the shade for her head-wear on her big day, ahead of yellow at 5/1, peach at 6/1 and purple at 7/1.
The Queen wore a pink hat on her 80th birthday in 2006 and the firm is offering odds of 8/1 that she wears the same shade to mark her 90th.
Outside colours include black at 25/1 and red or orange at 33/1. It is set at 100/1 that she ditches one of her favourite accessories and steps out of Windsor Castle without a hat.
Nicola McGeady, spokeswoman for Coral, said: “Betting on the colour of the Queen’s hat is the most popular royal market we have and with such a special occasion coming up this week, punters are lining up early to place their bets.
“If the gamble is right, it looks like the Queen will begin her celebrations wearing a blue hat.”
The nation’s longest reigning monarch will be out and about greeting the public on the day she becomes the country’s first nonagenarian sovereign.
On April 21, she will emerge from Windsor Castle, where she has been staying during Easter Court, and, accompanied by the Duke of Edinburgh, unveil a plaque for The Queen’s Walkway, at the foot of Castle Hill.
She will then go on a walkabout in the town centre, meeting the crowds who are expected to gather to sing her Happy Birthday.
In the House of Commons, Prime Minister David Cameron will lead MPs in paying tribute to the Queen.
The head of state - with the Philip, the Prince of Wales and the Duchess of Cornwall at her side - will later light the first in a chain of more than 1,000 beacons across Britain and the world to mark her personal anniversary.
Heir to the throne Charles will stage a private family dinner for the royal matriarch in the evening.