Internal-external grabber differences

I see big differences in the LEDcolors when using the calibration pictures and when getting a similar picture via the usb-grabber.
Is there a way to check which rgb-va!ues the grabber (STK1160 based) delivers ?
The screenshot looks quite ok, the ledcolors are not directly incorrect, but way off the expected color tone.
Example : with a grey calibration picture the leds are reflecting this very good, sending a grey picture through the grabber, the leds will get color, most blueish