[Transparency in Designer]

I am trying to effectively do a "bucket fill" with transparency on a pixel image.

 

The "Flood Fill Tool" does absolutely nothing for me (I have no idea how it works). The "Transparency Tool" only allows one gradient at a time. How can I use multiple instances of the transparency tool?

 

Is there a way to draw/erase/fill transparency, or am I stuck using only one gradient? The same issue occurs with vector graphics that I've drawn from scratch using Designer. I've been struggling with this for over an hour.

 

Any help would be immensely appreciated.

 

Thanks,

 

--Joseph