The upconversion luminescence (UCL) properties of Yb³⁺/Ho³⁺ (Er³⁺) co-doped or tri-doped LuPO₄ phosphors were studied and compared. To further improve the UCL intensity and control the emission color, optically inert ions such as Li⁺, Ca²⁺ and Gd³⁺ were doped into this system. Compared to the optically inert ions-free counterpart, the red UCL of LuPO₄:20%Yb³⁺/1%Er³⁺/1%Ho³⁺ with 5%Li⁺/10%Ca²⁺/10%Gd³⁺ doping is enhanced by 5.93 times. It is worth pointing out that the optically inert ions doping could not change the red-to-green intensity ratio. Therefore, the emission color was still maintained irrespective of the type of optically inert ions used for doping. Moreover, the reasons for the above experimental phenomenon were also discussed in detail.