When making copies or prints, there is a red "smudge" covering about 15% of the pages. It almost looks like colored pencil shading. At times it is more severe and at times it is less. Ink cartridges have been switched out and it does not fix the problem.

Imaging unit is the problem. It's made for 100k but often there is a problem very soon after replacing. There is a belt in this unit and he has a lenght enough for two pages A4. Thats why you have problem on every second page. Try to reassembled it and see what is wrong inside ot imaging unit.
If you ask me...buy new mashine. This one have a hundreds of problems by default.
Pifonski

Problem is either the cartridge or the fuser.
Put paper in the manual feed tray(tray 1)
Print something. When the paper gets 1/2 way into the printer, open the
top cover. Pull out the toner, look down at the paper.
If it has the smudge replace the toner. If no smudge, replace the fuser.

If this just started after you replaced the toner cartridge, one would have to say that it is a bad toner cartridge.
Print something that smudges. Pull down the front cover and look at the cartridges. Pay attention to the top one(black).
Look at the drum on that cartridge. You should see the toner sitting there. Compare it to the other cartridges which should be clean.

Run a 1/2 test.
Put paper in the manual feed tray(tray 1).
print something. And watch the paper. As the paper gets
about 1/2 way into the printer, open the top cover, remove the toner.
Look down at the paper. If you don't see smudged text, the fuser is probably bad.
If you see the smudging, replace the toner.

How is the printer printing black and yellow color? Is it a smudge?

When I print, the pages are smudged with ink. What should I do?
Description: - If smudged stains appear at the top center of the printed page:

• Make sure the paper is not too thick or curled.
  Also, verify that the appropriate paper is being used and that the paper type set in the machine matches the media used.
• Make sure the paper is not curled when you print on the back side of the paper with the Print Odd Pages and Print Even Pages settings.

- If smudged stains appear at the right or left corner of the printed page:

• Make sure the paper is not curled when you print on the back side of the paper with the Print Odd Pages and Print Even Pages settings.

- If smudged stains appear on the reverse side or at the bottom of the page:
NOTE: Illustrations shown below are from the representative product and may differ from your Brother machine.

• Make sure the printer platen is not dirty with ink.
  
  How to clean the machine's printer platen
  
  
  1. Unplug the machine from the AC power outlet.
  2. Lift the scanner cover until it locks securely in the open position.
  3. Clean the machine's printer platen(1) and the area around it, wiping off any scattered ink with a soft, dry, lint-free cloth.
     
     
  4. Close the scanner cover.
     
     
• Make sure you are using Genuine Brother Innobella™ Ink.
  > Brother Genuine Supplies information
  
  
  
• Make sure you are using the paper support flap.
  
  To use the paper support flap, pull out the paper support (1) until it clicks and unfold the paper support flap(2).
  
  

change your black toner... if it doesn't work.. then change black developer... still doesn't work then change black drum...

I’m happy to assist further over the phone at https://www.6ya.com/expert/norman_d2fa496dce30c65d

- Try to reallign the cartridge, run a print test, after that if the issue still persist - uninstall and reinstall software - run test and re-allign again the cardtridge using software. run print test again.

Are you scanning both black and white and color the same way? The ADF (automatic document feeder) uses a different scanning area than when you place the document on the scanning glass. If you open the scanner cover, look to the left of the main scanner glass to the narrow strip used for the ADF. Make sure this narrow strip is clean -- no ink or white-out or scratches. This can make your copies look dirty or smudged.

Test page printed properly. Don't know what to look for in the test page or the applications. the cartridge was changed recently

Keep on head cleaning until the test print will complete colors.