I've changed my colour ink cartridge three times now and it still wont print red.

Ink is a lubricant for the print heads. Some color ink is used even if you only print in black or vice-versa. If you printed with a color missing, you could destroy that print head from the friction of running without ink to lubricate it. That's why most printers won't allow you to print with a color missing even if you aren't using it.

If you allow the red color to go empty, the residual ink in the head could clog and become permanently blocked. You would never be able to print that color again. So always replace ink cartridges as soon as they are empty.

Leaving your printer on takes very little power. Every so often the printer sends a signal to cycle the ink in the heads so it doesn't clog. If you turn your printer off, the next time you turn it on it will do a cleaning which wastes a lot of ink. The excess ink is soaked up by the waste pad beneath the printer. When the waste pad is full, the printer stops working. Unless you want to spend more than the cost of the printer or waste a lot of time on a messy cleanup you will end up throwing the printer out. So shutting it off actually shortens its life.

If you leave the printer off for extended periods of time, the ink in the heads will clog and you will have to throw the printer away. Someone I know with a Canon (though this can happen with any inkjet) had to have the heads replaced twice within the first few months before the Canon tech told her to stop turning it off. Since then it has worked perfectly for over a year.

How to clear blocked print heads:
-leave the printer on for a day
-run several head cleanings and print the nozzle check pattern
-if that doesn't fix it, remove the cartridge(s) from the blocked head and drop alcohol where the ink enters the printer.
-reinsert the cartridges
-run one head cleaning
-wait about an hour then run some head cleanings.
-dip the print heads in Isopropyl Alcohol overnight, which you can buy from chemist
-if that still doesn't work, your heads may be clogged to the point where they need repair or replacement, which will probably cost more than a new printer.