How can I control Mail Message background color from Applescript?

The correct syntax, which still introduces a -10000 AppleEvent handler failed message is:

use scripting additions

tell application "Mail"
	if it is not running then activate
	if not (get selection) is {} then
		set thisMsg to item 1 of (get selection)
		tell thisMsg
            -- the other enumerator forces Mail to open a color chooser dialog
			set its background color to other
		end tell
	else
		return
	end if
end tell
return

When it is in the proper mood, this script will open a color chooser panel when it detects the other enumerator key word and will use your choice of custom color once set on that color panel. You cannot assign a color directly to background color as it expects an enumerator constant from the color panel, not from you.

Ibid, Urguhart1244.

Here is some AppleScript that I rescued from MacScripter days that allows you to pick a color on the color chooser and convert that to 16-bit RGB and hex color codes, that are presented in a dialog.

-- StefanK: http://macscripter.net/viewtopic.php?pid=95047

property hexList : "0123456789ABCDEF"


set defaultHex to "#FFFFFF"
set r to ""
set g to ""
set b to ""

if defaultHex starts with "#" then
	try
		set chosenColor to choose color default color Hex8toRGB16(defaultHex)
		-- log chosenColor
		set {r, g, b} to items of chosenColor
		set rgb16Value to "{" & r & ", " & g & ", " & b & "}"
	on error
		return
	end try
	set hexValue to RBG16toHex8(chosenColor)
	log hexValue
	
end if

display dialog "RGB 16: " & tab & rgb16Value & return & "     Hex:" & tab & "#" & hexValue
return

to RBG16toHex8(AppleRGB16)
	set hexString to ""
	repeat with primary in AppleRGB16
		set hexString to hexString & character (primary div 4096 + 1) of hexList & character (primary div 256 mod 16 + 1) of hexList
	end repeat
	return hexString
end RBG16toHex8
--
to Hex8toRGB16(htmColor) -- format #xxxxxx
	set RGB16 to {}
	repeat with i from 2 to 6 by 2
		set end of RGB16 to ((((offset of htmColor's character i in hexList) - 1) * 16) + (offset of htmColor's character (i + 1) in hexList) - 1) * 257
	end repeat
	return RGB16
end Hex8toRGB16

It set my selected message background to the other custom color that I selected, but only once and then that succession of -10000 AppleEvent errors followed. I recommend sticking to one of the predefined enumator color names.

Try using a different color notation:

set the backgroundColor to {65535, 65535, 0} -- yellow

That works very well, thanks!

Welcome.