I have to trim zero for a certain purpose. How to conceal donor area?

You can wear a hat after surgery.  If you are having issues after surgery surgery it is best to contact your doctor.

You may use concealer cosmetic powders or may have a scalp micropigmentation to camouflage the tiny white scars of FUE on your back scalp skin .

Your donor area is now what it will be. If the 'holes' from the FUE are a problem, you can have Scalp Micropigmentation to make them disappear, 

Whether you have an FUT or a linear FUT scar, careful SMP will add density and camouflage the scar.  The trick when you are camouflaging scar however, is to choose a practitioner of this technique who is VERY experienced.  Pigments that are used in tattoo ink are not usually suitable for this work because the scalp skin produces so much oil and is exposed to so much UV light that it fades to blue.
Good luck!