I know that there are mangas printed in color as opposed to black and white. One example of this is Colorful by Torajirō Kishi, which is the only one that I know of. What was the first manga that was printed entirely in color--as opposed to just cover pages on tankoban being in color?
In the Wikipedia page for "Manga", it says this about the history:
In 1905 the manga-magazine publishing boom started with the Russo-Japanese War, Tokyo Pakku was created and became a huge hit. After Tokyo Pakku in 1905, a female version of Shōnen Sekai was created and named Shōjo Sekai, considered the first shōjo magazine. Shōnen Pakku was made and is considered the first children's manga magazine. The children's demographic was in an early stage of development in the Meiji period. Shōnen Pakku was influenced from foreign children's magazines such as Puck which an employee of Jitsugyō no Nihon (publisher of the magazine) saw and decided to emulate. In 1924, Kodomo Pakku was launched as another children's manga magazine after Shōnen Pakku. During the boom, Poten (derived from the French "potin") was published in 1908. All the pages were in full color with influences from Tokyo Pakku and Osaka Pakku. It is unknown if there were any more issues besides the first one.
So in 1908, there was a manga called "Poten" that was printed in full color. There's more info about "Poten" here:
When we look at the history of manga, we see a side which was brought about by war. The Sino-Japanese War kick-started the Giga-Nishikie boom, and the Russo-Japanese War brought about the manga magazine publication boom. In 1905: “Jijimanga Hibijutsu Gaho”* (Kyoto), “Nipponchi”; 1906: “Tokyo Pakku”; 1907: “Osaka Pakku”, “Joto Ponchi”; 1908: “Ehagaki Sekai”, “Warai”, “Shonen Pakku”* and so on. In 1908, “Poten” was published in Kyoto.“Poten” apparently comes from the French word “potin”, meaning “rough-and-tumble”. Unlike other manga, it is 34cm long and 19cm wide, yet it has been strongly influenced by other manga magazines of the time. That the centerfold spread was a large single sheet of topical satire manga and that all the pages were printed in colour were influences from “Tokyo Pakku” and “Osaka Pakku”.Also, on the last page there are four picture postcards which one can cut out and actually use an idea borrowed from Miyatake Gaikotsu's “Ehagaki Sekai”. It is unconfirmed whether there are issues other than the first issue. (Shimizu Isao) (*)...Museum collection