If I understand you right, you like the stiffness and the washing will probably reduce that. However, you are afraid the color may run onto what you are pressing or ironing and washing will prevent that. Why not try ironing at the strongest steam setting and highest heat some scrap white fabric? If it doesn't fade onto that it probably won't fade onto anything. Then you are left with the decision of how you will fashion the canvas to your ironing board knowing that when you do finally wash it, it will probably shrink. If you can staple or tack the canvas down now and leave lots of extra you are fine. If you are going to cut it to fit then you will have to shrink it first. Good Luck.