Hello!
We already found out, how to switch the LED by buttons, thats fine.
My Ultimaker has settings to de/activate the LED during/after print.
What I want is that the LED lights during print (thats a std. Setting in Ulti), but after print, he should let the LED on for about a minute, because Octopod sends me on 100% a black picture, the Ulti immediately switches off the LED on finished print.
Is there a other sollution, that maybe Octoprint takes over the whole control on LED and I get what I would like to have?
THX
Hi 
What's your end gcode? I want to make sure there is nothing in there that switches the lights of.
Try if you can control the lights via terminal with M142 w255 and M142 w0
M104 S0 ;extruder heater off
M140 S0 ;heated bed heater off (if you have it)
G91 ;relative positioning
G1 E-1 F300 ;retract the filament a bit before lifting the nozzle, to release some of the pressure
G1 Z+0.5 E-5 X-20 Y-20 F9000 ;move Z up a bit and retract filament even more
G90 ;absolute positioning
G28 X0 Y0 Z0;move X/Y to min endstops, so the head is out of the way
M84 ;steppers off
the LED is actually controlled by the printer settings on the printer, can be switched off
M42 P8 S0 - 255 does control the led. did not try M142 jet.
thinking of, switching of the control on the Ultimaker, and make a pause in the g-endcode and then switch off the led, on g-startcdoe of course switching on ?
but when is the print done (100%), after ending the g-endcode?
Yup that's what I would do.
You could for example use a M190 R40 for the pause. This makes the printer wait until the bed cooled down to 40°C.
Just make sure to use a M140 S0 to turn off the bed later or it stays at 40°C 
(and you might want to remove the other M140 S0 from the endcode)