Thank you all!
1) Download Synthesia http://www.synthesiagame.com/
Configure your midi-controller/keyboard in "Settings" (music input/output) and look for the button "Free Play"
Hopefully Synthesia now use internal midisounds. And you can play in "real-time" and see the notes "fly away to the sky" Or use a stand-alone VST-plugin at the same time for better sound.
2) Open Band-in-a-box. Use same midi input as in Synthesia. I used a VST-plugin (Alicias keys) as Thru-instrumnet in Band-in-a-box (So pianosound is not from my external old digitalpiano) So bottom line.. I use Synthesia Free Play mode and Band-in-a-box at the same time. My pianosound (thru/vstplugin) and bossa-music comes from Band-in-a-box and Synthesia recognize my playing in real-time.
3) For screencapture, webcamrecording and sound recording, I use Bandicam: https://www.bandicam.com/downloads/
So just before I begin the recording I:
a) Start the song in Band-in-a-box
b) Make screen of Syntheisa visable
c) Start Bandicam recording.
These three steps I do very quickly :-)
This might also be useful when using VST-plugin:https://www.youtube.com/watch?v=R3HqE2EEcAs
I think this is the solution when you play a midi-file within Synhtesia and want better sound. For ex when you make your own tutorial. For different colours for left and right hands the midifile need two channels. Right/left hand. You can mute channel or change colour within Synthesia. More channels>>>More colours.
Here I´ve made a test playing Blue Bossa in free play mode and at the same time recorded it in Cubase. Free play mode:https://www.youtube.com/watch?v=N68yAeWaF-I
And here the saved midifile (seperated in to two channels) played in tutorial mode as a song within Synthesia using a VST-plugin for better sound:https://www.youtube.com/watch?v=GbyAaYlvhMw
And here is a another thorough example of a tutorial I´ve made:https://www.youtube.com/watch?v=d2eSm8mFVp0
(You can find different versions of this. Fast, slow, melody only etc...
Wow! What a long answer. Nevertheless I hope this is helpful. :-)