Natalie - as said, if you can get a material that's the colour you want in the first place, it does make it easy.
For black I either shoot against black velvet or use a black wall; for white it's a roll of paper... I have seen white material being used (sheets etc) but there's the issue of creases, that can create shadow, making it harder to light.
I rarely go down the gel route for creating coloured backgrounds, usually using coloured rolls or sheets of card. For smaller items where a piece of A2 card is big enough, you can get some wicked colours from craft shops like Hobbycraft.