Look at _ColorSetRGB, that will convert the 3 color values Red, Green, and Blue to a Decimal number. You can use either Hex representations of the 3 numbers or decimal or any combination of the 2 when you send it to that function.
This script was modified from the example script in the help file for _ColorSetRGB that shows what I mean. If you run it in SciTe you can see the value returned by the function in the console output pane of the editor.
#include <Color.au3>
Dim $aColor[3] = [0x80, 0x90, 255] ; I changed the last value from 0xff to the decimal equivalent to show that it works with both.
$nColor = _ColorSetRGB($aColor)
ConsoleWrite('@@ Debug(' & @ScriptLineNumber & ') : $nColor = ' & $nColor & @crlf & '>Error code: ' & @error & @crlf) ;### Debug Console
MsgBox(4096, "AutoIt", " Red=" & Hex($aColor[0], 2) & " Blue=" & Hex($aColor[1], 2) & " Green=" & Hex($aColor[2], 2) & @CRLF & _
"Color=" & Hex($nColor))