It is colour i need, my black ink is fisinh what to download from net

One can never recommend using non genuine printer ink as many are watered down and tend to splash around the printer . This can cause more problems in the long run. Pro dot is just a trade name. You can take the chance and you may be lucky. Its really a personal thing. I use Sainsbury's ink and have had no problems with it. It's up to you.

Some printers won't print unless both cartridges are present.
Find your manual here if you need to.
Good luck

Many printers especially Photo ones also use colour even when printing Black. so you do need to replace the spent colour cartridge.

If you only need to print in black get a B&W printer not a colour one.

Yes you have to have ink in all of them. No, accordign to Canon it wil not work without them. 

take the cartridges out and clean them also clean where they go with a lint free cloth then replace them.

Thats because it's using the other colours to mix what it thinks to be shades of grey. Due to the black ink being pigment based and the colours inks dye base, when printing onto glossy paper the machine will use the colour dye based inks to make up black. 
If you were to print on plain paper, set to greyscale, then the machine will just use the black as the pigment ink sits onto of the plain paper and will not run should the paper get wet.  With glossy paper the ink has to be absorbed into the paper ad the Black pigment ink will not do this so the machine will use the coloured inks.

Sucks but true. 

Here's a list of compatible cartridges for your printer.

I could not find the user's manual for your specific printer but you can go HERE to download one of the other 4000 series printer manuals. It's will probably be very similar to yours and give you the information you need to replace your ink cartridges.

First off you need to re-set the IC on the cartridge, for this you will have to purchase a Chip re-setter, google it on the net or check here for links

filling the cartridge is easy, but you need to get the right ink or you will mess up the printer head, normally when you order ink it will have the instructions, avoid kits that tell you that their ink is universal

The cartridge cannot be filled from the bottom, looking at the top of the cartridge almost directly above the were the ink comes out, you can see / feel an indent in the plastic label, stick the needle / filler in there - it should take about 6 - 8 ml of ink - you might find it better to try and fill the cartridge by facing it up, so that the ink can flow to the back, do it slowly, take a break if need be, you will be able to judge when it is full.

The Colour carts fill the same as the black, the colour code is correct on the original cartridge - Blue, Red Yellow (not like the sneaky HP carts)

Unfortunately, like most multi cartridge ink jet printers when one ink is empty the printer will cease printing regardless of whether you just want to print in black and white and you have plenty of black ink left. I have heard of people tricking the printer into believing the cartridge is full by placing coloured tape etc in front of the ink sensing device but I am not sure how the Brother senses the ink levels as i have some aftermarket ink cartridges for mine which seem to be completely non-opaque (unlike the originals). The need to have colour cartridges in even if you only ever print with black ink is good news for Brother as the MFC-210c uses a large amount of ink during it's routine cleaning cycles (that you cant stop) and will empty all 3 cartridges in under 6 months without ever printer a single drop of colour ink. Sorry about all the bad news, at least your not alone......