Patrice, the packaged, pretreated fabric sheets are washable only if the instructions say so. Some sheets are dry clean only (ugh). Depends on the company.
Here's what works best for me. Jacquaard cotton fabric sheets (ten per package at Michael's for $15.99, which I buy at 40% off with my coupon). I increase my saturation, brightness and ink output settings before printing. I air dry as well as heat set with a hot iron not once but several times over and over and over. Then I rinse in synthropol and when the fabric is dry, ironed and ready to be sewn, I spray with Wet Guard to make it water repellant.
The colors of the other brands of fabric sheets do not come out as vibrant on my HP.
I think I will try the new HP BJS. It will be less convenient than the sheets but a lot more economical.