I sometimes can't tell the difference between navy blue and black.
I can't always see the difference between navy blue cothes and black clothes.
この場合、次のような言い方ができます。
ーI sometimes can't tell the difference between navy blue and black.
「時々、濃紺と黒の区別がつかない」
can't tell the difference from A and B で「AとBの区別がつかない」と言えます。
ーI can't always see the difference between navy blue clothes and black clothes.
「時々、濃紺と黒の服の見分けがつかない」
洋服に関して言うなら、can't always see the difference between A clothes and B clothes「Aの服とBの服の見分けがつかない」のように言えます。
ご参考まで！
カナダ 
