To test any horse specimen for any color or colors the specimen might have is somewhat complicated, but I will try to simplify the answer into laymans terms. First the root bulbs are cut off the hair stalk, put into a test tube with a chemical that breaks open the cells that make up the bulb. The intracellular contents are then separated from the bulb tissue cell "left overs" in a process that involves centrifuge and chemical washes to separate the DNA from all the extra "stuff". The "raw" DNA is then put into small tubes and DNA replicator proteins and nucleotides are added to the tubes and then it goes through a process called PCR, Polymerase Chain Reaction, this is done by a machine, I will not go into how this machine does what it does because that is explaining DNA replication and that takes a chapter in a textbook. If you only want one color or a couple of colors tested they might not amplify the entire genome they may start with what is next to determine the color genes it has. The DNA is then subjected to certain color tests, and since you probably want to know all of the colors your horse could have then many different tests are done. These tests can be different but one way is some DNA is put into a tube with certain DNA primers, which bind to areas flanking the region of DNA for a certain color gene, DNA replication proteins and nucleotides are added and the DNA is then put through the PCR process, and the result is the specific color gene is amplified, if the horse does not have that gene it is not amplified. An assay is then done to the amplified DNA after electrophoresis of the PCR products, Electrophoresis is a gel with an electrical current run through it and the DNA fragments move a certain way through the gel to make specific bands that are seen by computer assisted laser detection of tagged amplified DNA fragments with fluorescent dyes or isotopic labels or other labeling techniques. All this is stored in computers and most testing is done by computer run machines where little human error or time is needed. Like I said there are other testing procedures it depends on which procedure works best for which gene which depends on its DNA characteristics.
Magic I find it interesting that your friend has a cremello horse and has a gene for perlino, since a perlino is a cremello horse two copies of the Ccr gene and the bay color genes of, agouti locus AA or Aa, and E+Ee at the extension locus for Bay, in other words the horse is either a cremello without the bay color genes or a perlino with the bay color genes but not have both and be a cremello. and if he is a cremello he can be pinto , even homozygous pinto but you would not be able to see the pattern, because of the creme genes, but if he was perlino you might if the patterns went through the mane, tail or forelock.
John