if you want to manipulate fixed-point values in FPGA you should check out IEEE.Fixed_Pkg from **broken link removed** it's a proposed implementation of fixed-point arithmetic for next ieee standard in VHDL, the library has some problems compiling under XST (Xilinx ISE, I don't know how 10.1 version of ISE works with this package) but works preaty nice with Precision from Mentor
if you doing color space conversion, I would strongly recomend google it for proper constants integer , if you are just looking at Xilinx app notes it is not enough