How to correctly color a row in a table in a loop through variables?

Question

Google Apps Script

Hello. I'm trying to color a string through this code in a loop

lr = sheet1.getLastRow();
  lc = sheet1.getLastColumn();               
  sheet1.getRange(lr, 1, lr, lc).setBackground('#ffff00')

But I do not understand why the range of lines grows with each cycle. How to correctly color only the line that you just filled out? Something with lr I do not understand.