Hi Mario, I'm Elio on astrofili.org, the italian forum you play
With this kind of filter there's no way, you must take a shot in a sunny day, when the sun is high (about 12:00) to a withe sheet, with your setup, like a flat but sun illuminated...then you keep there the rgb balance...
Try to see here,
http://www.astrofototecnica.it/Tecniche/coloredslr.html
your italian is better than my poor english, sorry
ciao
P.S. you can see here the difference on my wide-image taken with an uhc filter on 50mm lens
before wb balance
http://www.astrobin.com/46945/0/
then...
http://www.astrobin.com/46945/C/