(note: excerpt from page 181 of "The Century of Space Science", by J. A. M. Bleeker, Johannes Geiss, M. Huber, Published 2001)
The behavior of an artificial plasma cloud
The ionization can be observed not only spectroscopically but also directly with the unaided eye, because the barium cloud changes both colour and shape during the transition phase.
The non-ionized cloud radiates in several green, yellow and red lines of the visible spectrum. (image right from: new-england.net.eu)
After the initial, optically thick phase, the radiation in the green (spectrum) is the predominant colour in the neutral cloud.
As the cloud becomes fainter because of photo ionization, neutral strontium - always present as an impurity - remains. It ionizes much more slowly and radiates in the blue. (image left from optics.kulgin.net)
The ionized barium atoms have spectral lines in the violet, blue and red regions of the spectrum, resulting in a purple colour.
Hence the ionized cloud can be easily distinguished from the neutral one because the ionized cloud is purple and the non-ionized one is green and later blue. (image right from pictopia.com)