If the documents are editable (for example Microsoft Word) you could just globally change the font colour (edit menu - Select all, then select red or blue or whatever in the font dialog). However a problem with inkjet printers is that they use up ALL your ink even when you don't use all the colours. When you turn them on, and periodically after so many pages, they run a "cleaning cycle" to make sure the print head nozzles are not clogged. They do this by firing ink through all the nozzles. This means that even if you print everything in (say) red, the other colours will still run out, and if black is getting low it will soon empty and most likely the printer will then refuse to print anything, even though only one colour is empty. The business model inkjet makers use is mostly "sell the printers cheap and make money on the ink". When my girlfriend was doing her college course we kept running out of ink so I bought a Canon personal laser printer (black only) for the equivalent of 75 US dollars; we are still on the same toner cartridge 6 years later.
I have seen that some inkjet companies (Epson, for one) have recently brought in printers with huge ink tanks advertised to "last for years", for example the Epson EcoTank range has 4,000 claimed page capacity which they say will last "2 years". Of course everybody's usage is different. My Canon has a 3,000 page toner cartridge. After the college course finished we didn't use it so much, which is probably why that cartridge has lasted so long. We take digital photos to a shop to print out if we need to.
