Some armor that you try to dye is very hard to see the colors on the item dyed. This could be just the limit of the armor parts that can take a dye or not. Most armor has some cloth highlights which will show the color more than the metal parts. Also named weapons seem not to show off the dyes well, at least during my two or three playthroughs. I haven't been taking detailed notes. However I have seen some fairly significant colors changes in armor. You have a chance to forge a few really nice things in various places, so just try it.