A novel route toward tunable multicolor materials through phosphor‐in‐glass (PiG) technology is proposed in this work. Before that, an ultrastable Eu3+‐doped precursor luminescent glass frit without thermal quenching in the temperature range of 80–480 K is developed to serve as an encapsulant that not only protects the embedded phosphor but also provides the red‐emitting component for the PiG. By adjusting the mass ratio of Sr4Al14O25:Eu2+ phosphor to glass frit, a series of tunable multicolor Eu3+‐doped PiG is obtained and exhibits a good resistance to the harsh conditions. Meanwhile, the luminescent color of Eu3+‐doped PiG can be modified by changing the excitation wavelength or ambient temperature. Finally, corresponding Eu3+‐doped PiG encapsulated high‐power light‐emitting diodes are further fabricated, especially the warm white‐light‐emitting diodes (WLEDs), showing good color stability under different drive currents and with different periods of operating time. All these results indicate that Eu3+‐doped PiG is a potential color converter applied in the high‐power warm‐WLEDs and the route above opens up a facile and potential approach to obtain multicolor materials.