I have an old P3 laptop with an 800x600 screen on which I've installed WattOS 7.5.  I know it is now on 10 but 7.5 is the latest one that will fit on a CD.  The later ones will only fit on DVD.  This is an old machine - it only has a CD reader, it will not boot off USB and it doesn't like microWatt (which still fits on a CD).  There is something wrong with the microWatt drivers - the entire screen appears as a 0.25 bar.  Anyway, it has WattOS 7.5.My login screen is very reddish.  If I run it from my desktop it is greenish - which is the correct colour.  Also, for some reason, the system thinks it has a 1024x768 screen.  After logging in, all the colours are OK.The obvious answer would have been to ask the WattOS site but it is a chicken and egg situation.  It wants me to sign in and in order to sign in, I need an invite key.  I've got no idea what one of those is.  To find out, I'd have to login and ask but I can't login because I haven't got an invite key.Before committing the settings in stone, I decided to try them out.  First, I set the screen sizexrandr -s 800x600The screen size changes and I can now see the taskbar but the brightness has also changed to half.  When I type xrandr -q, I getxrandr: Failed to get size of gamma for output defaultFollowed by the resolutionsI suspect it is assuming I have 24 or 32-bit colour but this is an old system so at most 16-bit colour, which possibly explains why it has all gone dim and why my login screen seems to have lost its blue/green component.The questionsI must be looking up the wrong keywords - I can't find how to set the number of colours to 65536 or to tell the system that it has 16-bit colour.  All the hits I am getting are how to set console window colours.Another lot of searches says gedit /etc/X11/xorg.conf  This file doesn't exist on my system.  Again, I think I'm looking up the wrong keywords.  Almost all the hits tell me where it is, none of them tell me what has replaced it. How do I make these settings permanent so that my login screen appears in the correct colour and the system knows what size my screen is.Edit I've found https://ubuntuforums.org/showthread.php?t=1493835 Apparently xorg.conf will be used if created.  I'll give that a try later today.  The laptop can only stray from a power supply for 5 minutes: after that it shuts down.Edit This question is now purely academic - the machine just died.
Screen dims after xrandr
configuration
Finally got to the bottom of this.  WattOS has an on screen utility called arandr.   I've never been successful in pasting pictures so I'll do a sort of diagram.  On some systems, it is called the Screen layout editor.  On WattOS, it is called arandr.  I used it because I thought it had something to do with xrandr. ________________|____menu________||____icons_______||            |   || default    |   ||____________|___|What I was doing was going through all the menu options and icons and not finding anything to do with resolution.  The help doesn't tell you how to use it - all it has is the about screen.What I should have done is right click on the area marked Default, select Resolution and then pick the resolution.It is as simple as that: it has only taken me 7 years to figure out this one.