imperial, empyreal

Imperial or imperial describes something related to the government of an empire, the rule of more than one nation --
His Imperial Highness will be visiting our city next week.
Hundreds of people made up the imperial entourage.

Empyreal or empyreal describes someting related to heaven or the sky --
He wanted to write verse that would soar to empyreal heights, but it usually fell flat.
It was an empyreal blue, like the clear blue ether of the sky on a fine day.

Now that you know that, you can say --
"The imperial troops wore a uniform of empyreal blue; it was quite striking."