DrawText as black only

I am manipulating gray scale PDF, but when I add text, they become CMYk. How can I add text and retain "no color"? I only see SetTextColor for rgb and SetTextColorCMYK for 4 color.

I don't want invisible text. Perhaps I worded my erquest incorrectly. I want "black" only, so wen I pass my output through color separation, I do not have C-M-Y components, only K.

Thanks Ingo and Andrew for taking the time to respond. You steered me in the right direction.
I've now solved my problem. It appears changing pages resets the text color to a default 0/0/0 RGB black, which is not the same as 0/0/0/1 CMYK black for color separation purposes.

This does not work:

SetTextColorCMYK(0,0,0,1)
for each(Page in myDocument)
{
  SelectPage(page)
  DrawText(x,y,"sting")
}

This does:
for each(Page in myDocument)
{
  SelectPage(page)
  SetTextColorCMYK(0,0,0,1)
  DrawText(x,y,"string")
}