Sorry, I wasn't clear. When I say to take a reading immediately, I mean immediately after adding and mixing the pH reagent with the water sample. If I let it sit after adding the reagent, it will change to purple /false high; the higher the FC level, the faster the color change to false high occurs.

I've certainly never had a problem with the pH test at FC levels below 10 and am surprised you're having trouble unless you're adding the pH reagent and letting it sit.