Abstract: Methods for determining the presence of a ligand in a sample suspected to contain the ligand are provided, along with apparatus suitable for performing the methods. The methods depend upon a color visualization indicating the ligand's presence or absence in the sample. Preferred methods comprise contacting the sample with colored particles which bear on their surface a receptor specific for the ligand, passing the sample/particle mixture through a filter, and then analyzing the color of the filtrate. The presence of ligand in the sample is established where the color of the filtrate is substantially different from the color of the receptor-bearing particles.