testing runoff with general hydroponics test drops

Hello fellow ents. Ive been adjusting my water for a ph of 6.0 since the tips of all my seedlings have been turning yellow. I checked the runoff and just wondered if the discoloration of the water because of it going through the soil could cause the test to be off a little. I'm not using any fertilized soil just seed starting soil from walmart. thanks and criticism is appreciated.