From the Tools toolbar choose the desired Selection tool .Press F2 (or on the icon at the right side of the upper toolbar) and the Tool Properties window will open.
[Optional] For smooth transition between the black and white parts to the colorful part set a feather value greater then zero (The feather box founds on the Tool Properties window). Greater feather value will generate smoother transition.
Using the mouse select the area you wish to leave colorful.
Click on the Selections -> Invert menu (Or Ctrl + I) to invert the selection.
Click on the Colors -> GrayScale menu (Or Ctrl + F5 Or Effects -> Colors -> Black & White Photograph menu) to gray scale the selection.
Click 'Enter' to finish and remove the selection lines.
Select the area on the picture which is most similar (and preferably the closest) to the area on which the imperfection is located. To select the similar area point on it with the mouse and click on it with the right button.
In order to perform the repair, point on the area where the repair is needed with the mouse. Move the mouse on the area while holding down the left button to perform the repair.
Repeat steps 3-4 as much as need to complete the repair and to remove the imperfections.