Painting probes (sometimes referred to as Coatasomes or WCP's) are actually many separate region-specific probes that bind along a single chromosome, together giving the illusion of "painting" the whole chromosome.

Here is an example of a 19 painting probe. The normal 19's are the two right-hand bright yellow chromosomes. The leftmost bright signal is a portion of chromosome 19 attached to another chromosome. This test was used to confirm the identity of the extra material as chromosome 19 material.

Further information:

http://www.slh.wisc.edu/cytogenetics/procedures/fish/index.php