Responsa for Menachot 79:6

ולא יהא אלא לבן כיון דאפשר במינן לא

- Since one could use threads of the same material [as the garment], it is not [allowed to do otherwise];<span class="x" onmousemove="('comment',' Since genuine blue is unobtainable and in its place white threads are used, it is proper to use those threads which are of the same material as the garment, thus avoiding any clashing between precepts and obviating the one overriding the other; so that only white threads of linen may be used as fringes in a linen garment.');">¹⁰</span> this being in accordance with Resh Lakish's view, for Resh Lakish said, Wherever you find a positive precept and a negative precept [in opposition], if you can possibly observe both,<span class="x" onmousemove="('comment',' By carrying out the positive precept without at the same time transgressing the prohibition, as here by attaching white threads of linen as fringes in a linen garment.');">¹¹</span>

Teshuvot Maharam