I didn't notice any "red on black", so that problem must already have been fixed.
Two important changes you should make:
1. Have some way of erasing one of your entries, if it turns out to have been in error. At present the only way to erase your entry is to replace it with another.
2. Allow pencilmarks -- i.e. candidate entries. For example, if you have deduced that a given entry must be either a 4 or a 7, there should be a way to put both a 4 and a 7 in the cell (in smaller print). See http://www.kakuroconquest.com/
for a perfect example of how this can be done.