OK, I think I have figured out a way and like to share. This is, by no means, the only way but it is just working for me.
A review of my hardware with RGB LEDs.
a) Asus Z390i Gaming motherboard (still using the 1st release BIOS version 0401)
b) G.Skill Trident Z 2x8GB kit
c) MSI 2070 Gaming Z
First, I did not download/install G.Skill's RGB control software. Without it, the memory just syncs its colour with the RGB of the Asus motherboard. This is good as I do not want to install unnecessary software. RGB control softwares are known to have conflict with each other. So, if I can tame Asus, G.Skill will just follow suit.
Next, I enable Asus Aura in the BIOS. And, I set the colour effect, in Asus Aura, to static single colour. And, the brightness/intensity slider is slided to the left, making it zero light effectively.
Now comes a tricky part. Under Windows' Power Options/Plans', disable ''fast startup''. Fast startup will help a system with booting HDD to start faster but as my booting drive is a SSD, this does not matter to me.
This works so far.
I imagine, if you have other Asus RGB products, you can just link them all in Aura and it should work too.
But I have a MSI GPU which has its own control software (ie Mystic Light).
I install this software and use it to disable the LED. But it does not seem to save the setting and every time when the PC goes to sleep or restarts, the LEDs reset back to the default RGB.
I found out, by accident, that I need to check the 'SW Keep' option in its icon in the taskbar. I think this is supposed to save the setting. Maybe it means to keep the software setting?
So far it is working.?
Edit#1
Previously G.Skill just follows whatever the Asus motherboard does. So, I thought if I can 'kill off' Asus Z390i onboard RGB, G.Skill will just follow.
Now, I found out it does not!
I now have to install G.Skill's own software (ie Trident Z RGB Control) to play with and hopefully disable the RGB from within. Let's hope there is no conflict.