Wrong colour thread?
Or the thread is showing in the wrong place?
Nymo is brilliant for combatting this problem.
Using permanent, very fine nib pens, just colour the offending bit of Nymo.
Use a colour a shade lighter than the surrounding fabric at first. You can always go darker. But you can’t go lighter.
But do be aware that the colour will ‘bleed’ up the Nymo a ways. So just dot the pen a little at a time and wait before dotting again.