I don't think there is any way real way to change what you're getting at. People are always going to sell better looking items for more because there is more demand for them and they know that people will buy them. Because all of the stats are the same, people are going to place more value on the appearance of an item and not its effects... that's just the way it is and will be, there is really no getting around that. This is more something that can be influenced by the players than by the administration, IMO. If you don't want people to charge too much, then don't buy it from them or sell at high prices. I have a limited understanding of economics so I may be off base here, please correct me if I'm wrong. But regardless of that, I don't think anything can be done about more value being placed on different recolors of items.
About the Imperial Valk Helm itself: I'm with Ryuk that +5-7 stats is too much. IMO + all stats should really be avoided. We already have a lot of + all stats already, if you continue to add more to items, when new items are released they will have to be increased as well until one day you see +50 all stats on things like emp auroras and that is just a no. I don't think there is anyone who would be okay with that. People protested us doing this with Valkyrie weapons quite a bit and we were very careful about this, taking into account the strengths/weaknesses of each class, etc. and spent hours working on the effects. Not to mention these were only specific stats (eg. vit, dex, and so on) and not even all stats. All stats is just... scary.
Sorry for the lengthy reply, lol. In short: I don't believe that there's anything that can be done about the varying prices of items based on color and +5-7 stats is too much. I think it's fine the way it is.