Left panel: m_F275W − m_F438W, m_F814W CMD of NGC 1783. The black circles show the selected MSTO stars, while the red circles represent the stars identified as UV-dim. The solid green and dashed red lines indicate the MSTO ridge line and the selection threshold for the UV-dim stars (see right panel), respectively. Right panel: histogram of the verticalized color distribution (Δ(m_F275W − m_F438)) for the MSTO stars. The dashed red line represents the threshold in the verticalized color, Δ(m_F275W m_F438) = 0.16, used to select the UV-dim stars, which are highlighted in red. Gray and red Gaussian curves indicate the respective GMM fit components. The green line is the same as in the left panel.