Hello, my name is Vitor. I worked quite a bit doing modding Terraria at my free time. As I like it very much and I think some other do so, I'll show you quick steps to do like I do, not exactly , but fair. 
So, first think, you need to do a contour of a dark grey color a 9/100 scale of brightness of your weapon.
Choose few colors that you gonna use. Using to many color could ruin your artwork. I recommend you use less than 8.
While selecting the colors do a color palette based on the brightness and/or saturation to organize yourself. You also must have a least 4 tones of brightness in your palette.
Now it's time to paint. You need to organize yourself when drawing every part of the weapon. That's why I'll also recommend that you do a rough drawing in a paper before drawing in the PC.
After drawing, resize your image to be 2 times bigger.
The quality of the final result should look like this: [Example]
Of course, you can innovate as I did. This are just the basic rules.
