color sampling

[qwertylol]

While 1
MouseUp("left")
MsgBox ( 0, "title", "mouse is released" )

WEnd

I want to run msgbox only when the mouse is upped from down position.

but this will instantly report mouse button being up and thus not doing what I want.

how do I write it?

[therks]

IsPressed is your friend here.

#include <Misc.au3>
HotKeySet('{esc}', '_exit')

While 1
    If _IsPressed('01') Then
        Do
            Sleep(1)
        Until Not _IsPressed('01')
        MsgBox(0, '', 'Up')
    EndIf
    Sleep(1)
WEnd

Func _exit()
    exit
EndFunc

[PsaltyDS]

MouseUp() causes mouse up, it doesn't detect it. You want _IsPressed():

#include <misc.au3>

While 1
    If _IsPressed("01") Then
        While 1
            If Not _IsPressed("01") Then
                MsgBox(64, "Mouse Up", "The left mouse button was released.")
                Exit
            EndIf
        WEnd
    EndIf
WEnd