How to make Poinsettia pigment into ink?
I think that would be hard, you could try to use a mordant to convert a dye to a pigment, you can use Alumina $\ce{FeSO4}$  or $\ce{CuSO4}$. There is an article that explains it in detail how to use Poinsettia for colouring silk fabric.


  Rawat, Bhawana, et al. "Color fastness properties of silk fabric dyed
  with poinsettia leaves." MAN MADE TEXTILES IN INDIA 48.11 (2005): 418.


You can try to soak a thick paper in a solution of 1-5% of $\ce{FeSO4}$  with your dye heating gently. 

Another solution could involve a different medium, like arabic gum mixed with citric acid.