Hey there! I know this isn't what you want to hear, but honest to goodness observing objects and how light reflects off of them is how you learn how materials and textures react to light. But, that's like the whole "draw a circle - draw the owl" advice. It's just something you can take stock of to store for any future pieces.
With metals, you're looking at high reflectivivity. So light shining on it is going to be super sharp, bright, and vibrant. In contrast, the shadows are going to be dark and deep. Here's a small chart (specifically of pixel art) textures that might give you an idea:
The metal would be in second row, second to the right. So you see how dark the shadows are and how bright the highlights are? That's the contrast you need to create in order to accurately simulate metal and really make it pop. Also take a look at the lowlights, and how it's almost the same color as the base shade. Using techniques like this will make for a more believable metal texture.
For your sprite (which is adorable, by the way. Dogs in armor represent!), my first suggestion is to identify the light source. And according to your existing shading, that would be in the upper lefthand corner. I hope you don't mind but I did a small edit on your sprite to show you what I'm talking about.
So as you can see, I've placed a small light source indicator in the corner just to help me as I shade. Much like the textured spheres from above, I've used the same principle of high and low lighting to add a metallic contrast to the armor itself (very basically, mind you). I've used a dark grey-blue to define the outer-shaded edge, and high light grey-blues to make it stand out. I can see where you're having issues though, because your armor in this case isn't contoured out accurately, and that's going to make it harder to give it realistic texturing. I recommend looking at pictures of armor to help shape the body plates, since I didn't change any of the core design that you made, and I did find it difficult to give it that true metal feel. Go back in, shape things up nicely, and use the texturing techniques I suggested. I guarantee that'll give you the results you're looking for! :)
I hope this was a little bit helpful. Please post any future results so we can see your progress!