Typically, the colposcopist using something calle acetic acid which when in contact with abnormal cells will turn white. However, try not to worry as areas that turn white are not always due to the cells being abnormal. Scar tissue, sractches, abrasions, warts or any type of damage to the skin will also turn white. This is why they take biopsies so they can look at the cells under a microscope and decide if they are abnormal or not.
During my last colposcopy, I had an area which turned white and biopsies were taken and the result came back as no abnormal cells were found. Even if your results come back and abormal cells were found, it is very unlikely they will be cancerous. Pre cancerous cells are very common and depending on the grade of them, they can be removed sucessfully if caught in time. At my last colposcopy last year, I had abnormal cells graded CIN1 which are boardline and almost always go back to normal on their own. I've spent the last year worried about it to find out that they have gone back to normal and I am clear of pre cancerous cells.
What did the results from your smear come back as? Usually they are pretty accurate in detecting cell abnormalities, so this should give you a good idea of what to expect.
Try not to worry and remember that pre cancerous cells are NOT cancer and can be treated. I was told 4-6 weeks for my results but they came back in 3 weeks so you may hear sooner. All the best :) xx