Since I downloaded MACos High Sierra my computers colours and screen is corrupted and I cant view photots properly either I get a black picture

1. Generic troubleshooting- how to fix something that doesn’t work
2. My Mac don’t work – troubleshooting tools and techniques
3. Top 10 Troubleshooting Tips for Mac Startup Problems

Possible Fixes for High Sierra

You should try each, one at a time, then test to see if the problem is fixed before going on to the next.

Be sure to backup your files before proceeding if possible.

1. Shutdown the computer, wait 30 seconds, restart the computer.
2. Resetting your Mac’s PRAM and NVRAM
3. Reset the System Management Controller (SMC)
4. Start the computer in Safe Mode, then restart normally. This is slower than a standard startup.
5. Repair the disk by booting from the Recovery HD. Immediately after the chime hold down the Command and R keys until the Utility Menu appears. Choose Disk Utility and click on the Continue button. Select the indented (usually, Macintosh HD) volume entry from the side list. Click on the First Aid button in the toolbar. Wait for the Done button to appear. Quit Disk Utility and return to the Utility Menu. Restart the computer from the Apple Menu.
6. Create a New User Account Open Users & Groups preferences. Click on the lock icon and enter your Admin password when prompted. On the left under Current User click on the Add [+] button under Login Options. Setup a new Admin user account. Upon completion log out of your current account then log into the new account. If your problems cease, then consider switching to the new account and transferring your files to it - Transferring files from one User Account to another.
7. Download and Install 10.13.1 High Sierra Update
8. Reinstall OS X by booting from the Recovery HD using the Command and R keys. When the Utility Menu appears select Reinstall OS X then click on the Continue button.
9. Erase and Install OS X Restart the computer. Immediately after the chime hold down the CommandandRkeys until the Apple logo appears. When the Utility Menu appears:

1. Select Disk Utility from the Utility Menu and click on Continue button.
2. When Disk Utility loads select the drive (out-dented entry) from the Device list.
3. Click on the Erase icon in Disk Utility's toolbar. A panel will drop down.
4. Set the Format type to Mac OS Extended (Journaled.)
5. Click on the Apply button, then wait for the Done button to activate and click on it.
6. Quit Disk Utility and return to the Utility Menu.
7. Select Install OS X and click on the Continue button.

Select your internal drive. OS X Base System refers to an installer. You cannot install on it.

The problem may be due to the discrete GPU failing. Download gfxCardStatus 2.3 and use it to disable the discrete GPU. This is a temporary workaround until you get your motherboard replaced.

The above link will not work under Sierra in which case you should try using gfxCardStatus.app.zip.

Yes. Do the following with the installer app for the macOS version you wish to install. Or, you can erase the drive and restore the last complete backup of your old system.

Erase and Install OS X

1. Restart the computer. Immediately after the chime hold down the CommandandRkeys until the Apple logo appears. When the Utility Menu appears. Select Disk Utility from the Utility Menu and click on Continue button.
2. When Disk Utility loads select the drive (out-dented entry) from the Device list.
3. Click on the Erase icon in Disk Utility's toolbar. A panel will drop down.
4. Set the Format type to Mac OS Extended (Journaled.)
5. Click on the Apply button, then wait for the Done button to activate and click on it.
6. Quit Disk Utility and return to the Utility Menu.
7. Select Install OS X and click on the Continue button.

Kappy,

I want to thank you for the gfxCardStatus.app.zip pointer. I have a Late 2011 15" MacBook Pro with the discrete graphics issue, and I've kept it at El Capitan so that I could run the original gfxCardStatus. But the steveschow version will a) allow me to run it on newer macOS versions, and b) it seems to load locked into integrated graphics so even after a reboot, my old MacBook Pro will not slip into discrete graphics. As I have turned this older Mac into a home backup server, this is ideal, as it mostly works unattended.

Since this is not my thread, Please accept my "Thanks".