I took a picture in colour and while playing with some settings accidentally changed it to black and white. Before I knew what I did, it saved. How can I change it back to colour? Please help!!
If you convert the colour image, which is represented as w x h x 3 matrix with Red, Green and Blue "layers", into B/W image, which is represented by w x h matrix and and it is ovewritten, there is no way back.
It is like somebody irons your 3D origami into 2D shape. You have transformed 3D object to 2D object; you have lost one dimension.
The simplest algorithm to convert RGB image to B/W uses pixelwise operation:
b are scaling factors and
B are the values in the original matrix.
As you can see, if you want to reconstruct, say, the red layer, you have to know all the B/W image (you have it), scaling factors (you can find them, or expect them equal to 1) and both green and blue layers (which you do not know).
Dig your card, maybe there is some undeleter tool, if you shoot to both RAW and JPEG there is slight chance you have affected only one or you have set the camera to interpret RGB RAW image as B/W one.
If I were you I'd expect the worst and shoot the picture again, if possible.