Husky is correct, this is the case currently for QCAD but you can achieve gradient fills in some CAD packages.
This would make for a fantastic feature request...
...for QCAD too.
A Gradient tab could be added to the hatch dialog window and maybe offer one or two colours, then simply adjust a shade/tint slide. Already we use something similar when setting our background colour etc. in the user preferences so maybe something can be done for this in QCAD too
@andrew - what do you think? this would be such a bonus for making a very good 2D presentation drawing in QCAD.