Preferably free as i wont be using it much. I just need a programe that can change the colour of some of my pre-design stock colours. I have professional photos of some of my merchandise but want to change the colours of some dresses so I dont have to order one in every colour kind of thing just to take a photo of it for advertising. Can anyone suggest anything???

Photoshop is the industry leading Standard for 2d art.
however it's not cheap, for the sake that you are just changing clothes colours, you may want to look into a free alternative, like GIMP

NOT you can get a free trial of photoshop, in which case you want to open your photo,and a new layer,
pick a regular soft brush,
choose a colour change the layer style to 'colour' and then just paint the clothes,
you can then after change the opacity to make it look more realistic.
it really is SIMPLES!

EDIT: just added a quick video on youtube, link is in the Sources =].