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Theoretical color calibration is carried out for three (F140W, F220W, and F342W) of the most heavily used ultraviolet filters and one visual band filter (F480LP) in the Faint Object Camera on the HST, using the new Kurucz model spectra which became available recently. It is known that the UV filters suffer from the $visible$ $leak$ problem, and it is apparent in this study that the visible leak problem can cause serious trouble to analyses of observed data. We show here that some of the colors defined from these filters do not have a monotonic relation with effective temperature; and thus, the colors fail to indicate a single effective temperature in certain temperature ranges. This study shows that the problem can be solved by using one of the visual band filters, preferably F480LP, together with the UV filters. The expected model color-magnitude diagrams for a typical globular cluster with the HST ultraviolet colors are also provided.
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