Natural mineral red clay was used as the color agent in red lime of traditional palace buildings. However, due to red clay mining being restricted, iron oxide red synthesized in the industrial method was commonly used to replace red clay in the restoration of ancient buildings, which caused poor water resistance, cracking, peeling, and other that phenomena happened in red lime. In order to explore the cause of this phenomenon, in this study, red Jia-long lime samples were collected from the roofs of three halls (Zheng hall, Xiwei Hall and Hou hall) of Yangxin Hall in the Forbidden city and determined the main phase composition of the samples by X-ray diffraction (XRD). The results show that the main components of the samples are calcite, iron oxide and quartz, in addition to a small amount of albite and halloysite. Since the quite difference in chromaticity of the samples from each hall, it combined powder microscopic morphology and mapping test, comparing the shape and color of the microscopic particles and the element distribution of the sample section to determine the main color components of the red lime sample. The results show that the powder micromorphology of the samples in Zheng Hall and Hou Hall is uniform and full of dark red particles, accompanied by black or brown-yellow mineral particles, and the distribution of Fe in the samples is not uniform and is inconsistent with the distribution of Al and Si elements. However, the powder morphology of the Xiwei Hall sample shows light color particles accompanied by other mineral components. The distribution of Fe elements in the sample is uniform, consistent with the distribution of Al and Si elements. It is concluded that the raw materials and coloring materials used in red lime samples from Zheng Hall and Hou Hall are lime, red clay and iron oxide red. The raw materials and coloring materials used in red lime samples from the Xiwei Hall are lime and red clay. It is the first time that the identification method between red clay and iron oxide red has been proposed. Scanning electron microscopy (SEM) was used to explore the effect of color components on the micro-morphology of the sample, and it was shown that the structure of the sample containing only red clay was continuous and compact, and the two phases were poorly combined after adding iron oxide red, and there were pores. In order to further explore the influence of red clay and iron oxide red on the performance of samples, the simulated samples with red clay and iron oxide red were prepared respectively. It can be seen that the use of iron oxide red instead of red clay to make Jia-long lime has poor physical and mechanical properties compared to the Jia-long lime made with red clay.