In Topaz Studio 1 adding a Color overlay adjustment at 100% gave the result of a complete one colour image (which I used with masking to get rid of backgrounds I didn’t want).
When I add a color overlay, the default is 30% opacity. I can get a solid color sliding it to 100%.
However, there is one small caveat. I’m finding that when I click-hold and drag the opacity slider all the way to the right, the value will say 1.00 (100%), but it’s often/always actually something like 0.95-0.99. If I select a different filter layer and then come back, it will say 0.98 for example. Clicking in the value box and then using the arrow key to bump it up to 1.00 correctly sets it.
So in the color overlay, dragging to max still shows a very faint shadow of the original layer until I manually set the opacity to 1.00. This applies to all opacity sliders I’ve used, not just on the color overlay.