Interesting, and thank you all for the feedback.
So for the unit in hand, i.e. m328p+GMT177-05 (ST7735S), if the driver is not refreshing the screen (I/O=0V, I suppose) the colors are OK, but all the colors are "overdriven" when running the out-of-range of Vdd=3.6-100*I and I/O up to 5V-Vsat. (I didn't pay much attention to the screen when running the original firmware with 5V//3.3V=4V, though).
Nevertheless, according to the quoted specs I set Vdd=5-100*I, and that apparently solve this part of the issues, thanks.
Now the unit worth a little more, and I will play around with the settings trying to avoid removing the ugly yellow glue, cutting the box, and putting all back toghether without damage.