A new thermographic nanocrystalline Sr₄Al₁₄O₂₅:Mn⁴⁺,Tb³⁺ phosphor was developed, and the concentrations of both dopants and the synthesis conditions were optimized. The combination of the thermally quenched luminescence from the Mn⁴⁺ ions to the almost temperature-independent emission from Tb³⁺ provides a sensitive luminescent thermometer (S_R = 2.8%/°C at 150 °C) with strong emission color variability. In addition, a figure of merit for this luminescence thermochromism was proposed, as the relative sensitivities of the x and y CIE coordinates, which for this phosphor reaches at 150 °C S_R(x) = 0.6%/°C and S_R(y) = 0.4%/°C, respectively. Noncontact thermal imaging was demonstrated with this phosphor using a single consumer digital camera and exploiting the ratio of red (R) and green (G) channels of the RGB images, thereby confirming the high application potential of Sr₄Al₁₄O₂₅:Mn⁴⁺,Tb³⁺ nanocrystals for thermal sensing and mapping.