Working with the CSS button on draw a layered graphic item, give it a colour.
With the item selected use Item>Duplicate use 1 copy with zero offsets to create the new item directly on top of the original. Select the top item and choose another colour for it.
With the Page/Site selector on Page you will see these 2 newly created items in your item list on the LHS - probably called itemX and itemXa.
In the item list shift click each one so they are both selected. Then Item>Group. With them still selected choose Item>Actions>Rollover to apply the rollover action to the group.
Now if you look in the Actions inspector window (item still selected) you will see the settings for the rollover.
The first Tab (images) should show 3 items (something like item7, item6 and item6a) Next to these items are tick boxes. You can ignore the top item (which is the grouped item) but the other two are the 2 different rollover states.
Make sure that only one of each of these is ticked in both the Normal and Mouseover states and make sure that they are not both ticked in both. Pic above shows settings.