I think the black on the HPs that you can change out to get a 6 color printer are only 17ml compared to 42ml on their 4 color printers. It could bankrupt you in a hurry if you printed much text.
hp 45: 42ml $29.99
hp 15: 21ml $29.99

most 4 color printers from hp now use the 15. I think the 900 series printers were the last to use the 45.

hp 56: 17ml $19.99

so.. the black ink is actually cheaper for the six color printers.