In this paper, we propose a thermally stable multi-color phosphor-in-glass (PiG) for chromaticity-tunable white light-emitting diodes (WLEDs). The multi-color PiG has a red, green, and blue (RGB) phosphor glass layer, which was screen-printed and co-sintered on a glass plate with a low temperature of 600°C. The WLED with ultra-compact structure was fabricated by the RGB-PiG directly bonded on a flip-chip ultraviolet (UV) LED. By controlling the weight ratio of R∶G∶B phosphors, the thickness of the RGB layer, and the weight ratio of phosphors to glass powders (PtG), the chromaticity of RGB-PiG based WLEDs can be effectively tuned with high color quality. With the R∶G∶B ratio of 2∶3∶2, the RGB thickness of 75 μm, and the PtG ratio of 2∶1, the RGB-PiG based WLED achieves a natural white light with a luminous efficacy (LE) of 27.8 lm/W, a correlated color temperature of 4245 K, and a color rendering index of 92.6 at the current of 350 mA. The LE losses of the RGB-PiG based and RGB phosphor-in-silicone based WLEDs are 4.7% and 14.6% after the aging tests of 1000 h at 100°C, respectively. The results demonstrate that the multi-color PiG is a promising converter for UV-excited WLEDs.