The story starts with a flashback to when Ken, Yuki, and Hiyori were children--Ken gets a ring from a toy machine, but is bummed because it's not the yo-yo he'd been trying for. Hiyori asks if she can have it instead, and Ken says OK, if she can get the yo-yo to trade with him for. Later on, Hiyori and Yuki try their luck at the machine and she does get the yo-yo, after a few tries--but she can't give it to Ken and asks Yuki to instead. Yuki does trade the yo-yo for the ring with Ken, but before Yuki can give the ring to Hiyori, Hiyori's moved away without a trace. Years later, Yuki and Ken are in high school, and one night Ken hears a voice from the closet--it's Hiyori, but how she got there, and exactly what happened to her since she moved away, is a mystery...
The storyline is short but interesting, with an air of mystery that reminds me of Kanon's, and with just two episodes to work with, they dispense with the side character fluff, the beach episode, the onsen episode, and the other staples of the genre, instead focusing on Ken, Hiyori, and Yuki. The characters are all likeable--Ken's your typical male lead, but he shows spine when he has to, which puts him in a class above most; Yuki's the not-related-by-blood imouto (makes you wonder, are any brothers and sisters in Japan actually related?); and Hiyori's more than a little ditzy, a bit of a crybaby, but means well. The supporting characters get a little screentime, but they aren't really the focus and given the length of the series it's just as well they just get more or less cameo appearances.
The animation and backgrounds are all really good--while it won't have your jaw hit the floor like some of Kyoani's work, it's still a good example of what "OVA quality" is supposed to mean (and sadly not all OVAs live up to). The voice acting is solid too, Yuuko Gotou (Nanami in Lamune, Asahina in Haruhi, among others) does a good job as Hiyori, although she has that kind of high-pitched voice that would maybe annoy some people. The character designs are what really stand out--the borders for the characters faces have thicker lines than some may be used to, but they really look good. If there was an award for the most kawaii character designs in anime, I'd be very likely to nominate this series--the character designer also did Iriya no Sora, UFO no Natsu's designs, so if you've seen that you'll know what to expect. The background music fits the series well too, there's just not much to find fault with.
Overall, if you like romance/bishoujo series I think it's likely you'll like Mizuiro. Part of me wishes it was longer, and that it ended with Ken making a choice on who he liked, but it was still a good, solid story, and although it was short it also helped to keep the story focused. Beware that like Pia Carrot, there's a hentai and a non-hentai version--"Mizuiro 2003" is the non-hentai version that I'm reviewing here, but there's also a hentai OVA that came out before it.