1-You need edit the UV Maping of the cube (in Blender for example)
2-Then asing a material to the face you want change the color(in Blender too)
3-Create a dynamic material (in unreal)
4-Asing the matrial to the cube face
5-Change de matarial color using a material dynamic instance when you want
6-DONE!!
You can use a line trace to do it
Do not use it… its answers are random… AI is not intelligent it is just artifitial