The structural features of colloidal monolayers formed from dilute silica suspensions dried on a glass substrate have been studied using optical microscopy and image analysis. Addition of small amounts of salt resulted in drastic changes of the monolayer structure. The quality of the film structure was quantified by determining the size of the hexagonally ordered domains from the pair correlation function and the defect population using an analysis based on Delaunay triangulation. The size of the ordered domains decreased exponentially with increasing amounts of added salt (0-2.9% NaCl/Silica ratio), and the concentration of large defects increased, suggesting that loss of colloidal stability and particle adhesion to the substrate inhibit rearrangement and ordering. The evaporation rate was controlled by varying the relative humidity; we found that colloidal monolayers with the largest ordered domains and the lowest concentration of stacking faults were formed at intermediate relative humidity (55% RH).