In Book 1, Ramona is first seen with her hair cut short all around, except for two long front pieces on either side of her face. As suggested by a piece of artwork, Ramona's hair is dyed a pale green, but in the color edition of the first book her hair is magenta. This is likely because most people knew it as pink following the release of the movie, and the color edition mostly used the same colors as in the movie because it would be more confusing to fans otherwise.
In Book 2, Ramona arrives at Scott's apartment with her new hairstyle. This time, her hair has grown out just a little, but the longer side pieces have been cut to about chin length, making the length of her hair smoother. Her hair is two colors, pink and a darker color as shown on the cover of that volume. either the darker color is lower layers of her hair that have grown out and had the dyed ends cut off, or this is the only hairstyle that involves two dyed colors as opposed to just one.
In Book 3, Ramona still shows off her Book 2 hairstyle halfway into the story, being that those two volumes are direct continuations of one another. Near the end, however, her hair is cut once more, but it derived from her second haircut with only slightly longer hair parts on the sides. This hairstyle is dyed in only one color, however it is not specified as to which color exactly (though signs point to it being brown). The only other clue to her hair color is when Scott asks Ramona if that's her "natural colour," in which Ramona replies that it could be. Adding to the humor is a caption box stating "NOTE: THIS BOOK IS IN BLACK AND WHITE." In the colour edition, Ramona's hair is navy, accompannied by the caption "NOTE: This was funnier in black & white". There is also one colored picture of Ramona with a similar hairstyle which is colored a brownish violet on the official Scott Pilgrim website, though in Free Scott Pilgrim (where her hair looks the same as in Book 3) it seems to be colored blue.
In the above gif, her hair is red at this point. O'Malley stated that they did not use this color because it conflicted with Envy's hair in that volume.
In Book 4, two months following the events in Book 3, Ramona has a new wavy hairstyle that ends up becoming the primary hairstyle in the game. Thanks to the volume's special splash page, we all find out that her hair is dyed a bright sand green, much like the color of Ramona's hair in the final scenes of the film. Also, in one scene with Scott and Ramona eating dinner, she claims that she will let her hair grow longer and eventually dye it again.
At the end of the color edition of volume 4, during the farewell party for Lisa Miller, her hair is dyed light orange.
In Book 5, Ramona's hair is long (mid-length) for the first time in the series. As stated back in Book 4, Ramona has simply let her wavy hair grow a bit longer, still retaining the basic style. Her hair is dyed blue, as clearly seen by the cover art of that book. Before her disappearance towards the end of the book, Ramona cuts her hair again in a manner similar to her first hairstyle (minus the long side pieces). It is unclear whether or not her hair was dyed another color for that style. O'Malley once colored it forest green, however.
In Book 6, the story continues with Scott without Ramona, being that she "disappeared" during a head glow near the end of the fifth book. However, after Scott is killed by Gideon, Ramona returns to see Scott in Subspace. At this point her hair is basically the same, wavy, mid-length style she had in Book 5, but now with the addition of a star-shaped hairclip and slightly shorter. According to the cover art of Book 6, her hair is dyed a bright sand green, much like the color of her fourth hairstyle but somewhat lighter. Ramona continues with this look for the rest of the book. In Book 6 it is also revealed that when Ramona was first dating Gideon, she sported a long, pale pink 'do, before cutting dramatically upon leaving him.
Ramona's hair in the movie is the same as its appearance in Book 1, albeit slightly longer, but still maintaining the two braids. Her hair starts out dyed pink, but during the course of the movie, she dyes it to blue, and then green for the remainder of the film.
When Scott asks Ramona about her hair (after she dyes it blue), she states that she "changes (her) hair like every week and a half" (as opposed to three weeks in the comic series).