そのウィンドウの、ハンドルかデバイスコンテキストがわかれば取得できます
#uselib "gdi32.dll"
#func GetPixel "GetPixel" sptr, sptr, sptr
#uselib "user32"
#cfunc WindowFromPoint "WindowFromPoint" sptr, sptr
#func ClientToScreen "ClientToScreen" sptr, sptr
#cfunc GetDC "GetDC" sptr
repeat
dim point, 2
redraw 0
color 255,255,255: boxf: color: pos 0,0
hw = WindowFromPoint(ginfo(0), ginfo(1))
mes "ハンドル "+hw
hd = GetDC(hw)
mes "デバイスコンテキスト "+hd+"\n"
ClientToScreen hw, varptr(point)
sx = ginfo(0)-point.0: sy = ginfo(1)-point.1
GetPixel hd, sx, sy
r = stat & $0000FF
mes ""+sx+" "+sy
color r,,: mes "R = "+r
g = (stat & $00FF00)>>8
color ,g,: mes "G = "+g
b = (stat & $FF0000)>>16
color ,,b: mes "B = "+b
redraw 1
wait 20
loop