Ah Ok, as I remember the TH defaults to auto white balance, Gorgeous Mode, Auto exposure, 4K @30FPS.
None of those settings are great
4K video requires quite a hefty computer to play back at full frame rate - the files are so huge, and the data rate very large, so lesser machines will struggle.
The answer to that is to record in 1080P at either 30 or preferably 60 FPS (this allows you slow footage down in a 30FPS project file with no loss of frame resolution). 1080P still looks good on youtube, and can be played back without framerate issues on most modern computers.
Now, the Auto settings, and why we don't use them...
If you keep WB on Auto, and don't lock it, you will find that every time your camera is pointing at the sun, the whole image will colour shift more towards yellow, and back again when you turn away, pretty much ruining most video you take in that mode ! You may also notice short white flashes, which are also only present in this mode.
Exposure settings are also important, and again manual wins the day every time - by clearly exposing for the sky OR the ground, not some hazy average of both you can get clear professional-looking footage from that camera.
There are videos that go into the practical setup of all that...here's a good one...
I should mention that before you setup any of the camera controls, bump a D-pad (once up, once down), which resets WB/Exposure the first time you do it after system power on, THEN setup your camera before launch. Do White Balance on the ground, and set manual exposure and rough exposure setting that is mostly right, then when in the air, point the camera at what you are exposing for first (normally the ground) and change the exposure setting until it is correct, then begin the flight. When you want to include the sky in shots, point the craft at a bit of sky 90 degrees round from where the sun is, stop and hover, adjust exposure way down so the sky isn't whiting-out and flaring. then resume the flight, stopping again to set it as often as required based on when you change the gimbal tilt. That approach will give you the maximum amount of usable shots for your time in the air.
As for why stuff might be grainy - this happens when light levels are low, or ISO is set too high. The CGO3+ doesn't like low light conditions, and does least well in them, however with all-manual settings can give decent results even then...
Hope that helps...