Jalview features
The letterconf command can generate a features file for the popular Jalview alignment viewer and editor.
In Jalview, first load the alignment then load annotations as shown in the screenshot below.

The feature file format is documented here: http://www.jalview.org/help/html/features/featuresFormat.html
The first ten lines of the feature file specify the colors for letter confidence values 0 through 9. Colors are specified as 6-digit hex values RRGGBB, as shown below. You can change the colors by editing this file.
LC0 A00000
	LC1 902020
	LC2 803030
	LC3 703030
	LC4 603030
	LC5 404040
	LC6 407040
	LC7 308030
	LC8 009000
	LC9 00A000
	STARTGROUP Muscle5_LetterConfs
	- 1o20_A 0 1 54 LC9
	- 1o20_A 0 55 55 LC6