Yeah, it's okay if you use those in a mod. Also, yeah! That looks much better with the darker outlines and increased contrast. You might wanna go for more contrast in the sand parts of those weapons, though. Are those the vanilla palettes for copper and sand?
I think @KittyKitCatCat means something like this. I tried to improve your stuff a bit, but as you can tell the axe and the hammer are a bit difficult for me xD
If you notice, I increased the contrast between the colors, used colors that stand out more, and made the outlines much darker.