I can't understand that I mean their hair length & color is different their voices are different enough as well so aside from the face it should be easy enough to tell them apart from just those 2 things they're big enough clues if you ask me.
They all have the same hair color actually. That's just a visual thing to make their designs more interesting when colored in. In the manga, you can't tell their hair color apart. Their difference in voices also only exists in the anime. They're actually supposed to have similar voices.
So in the manga, there actually are multiple times where you can't tell them apart. Especially during a specific part where they all wore the same hairstyle.
There's still the hair length & hairstyle which once you've seen them all in the same room I think someone as smart as Fuutarou should be able to tell them apart & see through a simple hair accessory trick.
Which specific moment where he mixes them up are you talking about anyway? The only moment I remember was early on when Miku wore Yotsuba's ribbon. But Futaro was in a hurry then so I think it's kinda understandable. Every other time he mixes them up, they were copying somebody else's hairstyle and/or wearing a wig.
Anyway, just chalk it up to suspension of disbelief. lol. They're probably harder to tell apart in-universe.
There's the time when he's let into the house he confuses Miku for Nino while she's in a towel drying her hair, then the time he confused Ichika for Miku when he stayed the night & slept in Miku's room & the time you mentioned. Otherwise when they aren't using pour impersonation tricks like just using a ribbon & nothing else & don't try to trick him for what ever reason he manages to tell them apart, so I'm going to maybe go with the answer that perhaps its just the writer cherry picking when he doesn't have the capability to tell the difference between the 5 of them.
The towel thing, Nino's hair wasn't done. Nino and Miku both have long hair, so they could be mistaken for each other at first glance. He realizes it's Nino as soon as she fixes her hair
I don't recall him confusing Ichika and Miku when he slept over though. He pretends that to think that Itsuki was Miku so that Itsuki would talk to him.
But again, just suspend your disbelief whenever people confuse them lol. I think it's much more effective in the manga though. Granted I haven't seen season 2 so I don't know how well they adapt some of the quint-confusion from that part.