Process of Vector Low poly Art:
1. Create a new layer under the image layer.
2. create triangles shape with Pen tool.Fill color should be none
3.After completing triangles, select all Shapes and Convert to Live Paint Object
4. Pres alt to activate Eye dropper tool while you are working with Live Paint Bucket tool. Release ALT to Activate Paint Bucket tool after select the color. Then Fill with paint bucket tool.
5. After Filling all shapes, Select the whole object and apply Stroke Color None.
Thank you