Problem of non-uniform color

system : win10
UE version : 4.16
Hello everyone, I want to generate the masks of trees in a forest scene to create a dataset for segmentation. To achieve this I replaced every material of each trunk with a new material with a unique emmissive color which was assigned fix values of R,G,B. However, the result I got is within the pixels of a trunk, the color is not uniform (not same at all pixels). I thought it was light, but after I turned off all lights in the level the problem still exists. Does anyone know where the problem is?