These pages are using spot coloring:
This type of printing consists of printing with 1 or more pre-mixed colors (generally 1, 2 or 3 colors at once). Spot printing uses a color system of solid, premixed colors known as Pantone colors.
As mentioned on that page, the benefits of spot coloring are:
When using spot printing, colors are more accurate because you have already determined the color by its specific Pantone number. Since the ink is not being mixed, that color will never change from one print run to the next. For example: a Pantone 186 Red will always be 186 Red. Some colors, such as metallics and fluorescents, are only available as spot colors—the same color and effect can’t be achieved using 4-color process.
Spot color is generally more economical if printing is kept to 3 colors or less.
It is nowhere explicitly mentioned why spot coloring is used for manga. However, it can be said that they're used as an eye-catcher. Usually single spot color pages are used to make the publication feel more colorful but not pay the full price of colored pages.
In Japan, where Manga are published in magazines, such pages serve the purpose of catching the attention of the reader without costing the publisher as much money as it would have cost to publish a full-colored page and nor the artist/mangaka extra time to color each part of the page separately.