It is controlled by a Cell option called
EmphasizeSyntaxErrors. It can be removed by fixing the error and rerunning the cell, or manually using the cell expression or option inspector.
To view the expression of a cell, put the cursor in the cell or select its cell bracket and press Shift-Ctrl-E (also available under the Cell menu). Here's both the cell with an error and its cell expression:
Then just delete the
, EmphasizeSyntaxErrors->True (don't forget the comma)
or change it to
Alternatively, select the cell bracket and go to the Option Inspector (Shift-Ctrl-O or under the format menu) then under the "Editing Options" section you'll find the
EmphasizeSyntaxErrors option that you can delete/set to default (on the left hand side) or set to False (on the right hand side):