Am I doing it right?

I just found a formula error and fixed it. Why did I lose all my progress?

Yes, the testedness bar went down and some cell’s borders turned red, but that’s what’s supposed to happen. When you change a formula, the decisions (checks and Xs) that you made for old values can’t be used anymore.

All the cells turned red when I fixed a formula error. Does it mean the cells are wrong?

No, the red colors just mean that the cells aren’t tested anymore, because the formula you changed affects them. Make decisions about their new values. If you decide the values are right and check them, the border colors change again.

Why do some cells turn purple when I check them? Aren’t they supposed be turning blue?

Blue means the cell is completely tested, for all situations. Purple is good too, but with purple there are still some situations left that need testing.

What is a ‘situation’?

It’s a relationship between part of a formula and parts of other cells’ formulas that it refers to. For example, a cell’s formula can have two cases, A<B and A>B. And A’s formula might also have two cases. If we check a value in one case (A>B), that situation gets more tested. But there is still another case (A<B) that has to be tested, and these relate to the cases in A. The arrows show all the situations. Testing new situations is how you make progress in testing.

When I place an X-mark, I get so many colors. What color should I focus on?

The colors are just to help you keep track – so you can be systematic with finding errors and testing the cells. Your goal is to find the error and fix it, not to change the colors. The cell interior colors just help you narrow down where the error is and the border colors just keep track of the amount of testing the cell has had so far.