In clinical pediatric dentistry, a base material with optical properties, including transparency, that can mask the color of the material used for root canal-filling is preferred. This study aimed to examine the optical properties of various base materials by thickness. The disk-shaped specimens were photopolymerized and fabricated using Ionosit (IN), TheraCal LC (TL), TheraCal PT (PT), and A2 shade of Filtek<sup>TM</sup> Supreme Flowable Restorative (FZ), Fuji II LC (FL), and Ketac<sup>TM</sup> Fil (KF) with 1 and 2 mm thickness. The color parameters of these specimens were measured using a spectrophotometer on a black and white background and were measured using the same method on a mold containing Vitapex® and gutta-percha. The translucency parameter (TP) and color difference were calculated for each group. The Kruskal-Wallis and Bonferroni tests were used in the statistical analyses. The TP decreased when the thickness was 2 mm compared with 1 mm. The TP values of TL and PT were the lowest at all thicknesses. The TP values of 2 mm thickness in all molds filled with Vitapex® and gutta-percha were the lowest for TL, PT, KF, and IN. In TL and PT, the color difference before and after the application of the canal-filling material was the smallest, regardless of material thickness. Within the limits of this in vitro study, TL, PT, KF, and IN demonstrated better masking of the color of canal-filling material.