This is a nice sword, but there's actually 13 colors- link
I looked at it up close and it seems like you used a few colors that aren't in your palette at the top. For example the pure black outline isn't in the palette, or the grey outline. There's two greys in the blade of the sword that also aren't in the palette, and one pixel near the base of the blade on the right side that is the only pixel with that color.
Other than that it's a pretty good try at making a sword.
This didn't accumulate the score required for the gallery.
my analysis of why: the sword is too simple, too basic. that's not a very comfortable looking handle. The handle is too thick compared to the guared, which is too rectangular. the blade is dull and blunt, it's rendered more as a rectangle with a pointed tip than the sharp blade of a sword.
You need to use references. What type of sword is this? Historically, there have been different styles of swords used in different cultures, and they all have their own distinct appearances. this seems like a generic sword, reduced to geometric forms, which makes it somewhat more like a symbol of a sword than a depiction of a sword.
Having the palette in the piece is also draggin' it down, have you ever seen a painting in an art gallery with a palette in the corner? Since this is for your gallery, it's usually best to disclude the palette from the final piece.
Shading seems absent. It seems like it's somewhat pillowshading, but at the same time covered all over with a dither, and none of this is defining the volumes of the sword. Think about how light hits it. Use less colors, more contrast (as well as a little saturation in your greys, pure grey is kinda bording. The browns might could be a little more lively as well, warmer tones)