Begun in 1787, Curtis's Botanical Magazine is still being published, making it the oldest periodical in existence featuring colored plates. Two sites host electronic facsimiles of the early volumes. The University of Georgia Libraries has the first 20 volumes (DjVu format) online in full color. And the National Agricultural Library has many images from the first 26 volumes. Introductory information can be found here.
University of Georgia Libraries
National Agricultural Library
Glasgow University Special Collections
Botanical Wunderkammer, Part II