With nothing selected and the rectangle tool active, set the color and fill you want.
I did that, but when I made the box it still changed it to gray and black.
Bob is right, make sure you have no element selected (Edition > Unselect all)
Although I'm not sure you must have the rectangle tool active. I thing selection tool selected would word too... anyway...
Then change color fill and stroke as desired : no border and white or blank fill.
Finally create a new box. It has to work... ^^
are there any object styles into play here?
Open your Object Styles panel.
Watch out for this little icon that defines what's the standard for fill and stroke (and other things) with new rectangles:
After someone dragged that icon down to a different object style and clicked in the area below the last style listed:
Every time you'll draw a new rectangle, the object style will dictate its properties: