Suppose you have to fix a corner like this:

A
X A

So you want to change color X to A, without changing anything else at all. Well, use the...
First, see if you can find a cell in X's column which is by itself in a row (call it Y); similarly, find a cell in X's row which is by itself in a column (call it Z).

That is it: click YXXZ, and the only cell that changes colors is X! If X is still not the desired color, do it once more, and eventually X will match A's color.

So, if you had:

. Y .
. A B .
. X A Z
. . . .

(Again, make sure Y and Z are by themselves in their row/column... Or at least make sure you do not care what happens to Y's row and Z's column)

You will end up with:
. Y .
. A B .
. ? A Z
. . . .

Now, if everything is perfect, except for that one lonely cell, like this:

AAAAAAA
B.......
......

In this case, use the...

It is simple: let Z be the cell right above B. Find a cell in A's row which is alone in its column (or whose column you do not care about at this point), call it Y. So I mean this:

ZAAAAYAA
B.....
...

Now click YYZ, and B is the only cell which will change colors! If B still does not match the color A, do it again and now you have:
AAAAAAA
A.....
....

as desired.