Well, what I do is move the color up stubly each time you overlap to an almost white shade. It also helps to make the lines you draw a bit transpaernt. I hope this helps because I am terrible at explaining....
There are all kinds of ways to make things look shiny.
I worked for a long time drawing new and shiny things for media players. I did chrome, brass, shiny plastic, glass, and wet stuff. I did other things like neon lights, glowing things, and sparkles of different kinds. It took me a while but I think you may have already did some.
A colored sparkle like lens flare- You draw a color spot then draw a smaller white spot inside with a soft brush. Then you use the Smudge tool about the same size as the white and drag a line out in different angles to make it look like a sparkle. Add other sparkles different sizes and different amounts of work done with the smudge tool to make different stages of sparkles.
Metal flake paint - using the right colors for metal flake can make it look like metal flake in a distance, but close up you might wand to draw the painted object then do another layer with the brush with the brush Mode set on Dissolve. That setting is usually on Normal. Practice with the flow settings so you can paint splatter in one stroke or use a lower flow to do less to control the coverage. Once you have the metallic splatters, hold Ctrl and click on that layer so you can see it is selected. If I'm wrong then it could be the shift key. Start a new layer and hide the splatter layer. The new layer you need to paint a reflection color if you have something of another color close by in the shadow area. Then paint a light shade of the color to make it look like bright specks under the paint. Then on the highlight area paint that white to be the brightest part of the metallic.
Metal, Plastic, and many other shiny surfaces needs a reflection to make you say that is shiny, You can copy the floor or surface it is on and paste it back in and flip it with Transform. Now you need to fade the reflection and get it in the shape of the object. If it is a ball you want to use Filter/Distort/Spherize. Anything you need to get it to fit the shape even trim the edges or use the eraser set on brush, the brush with the Mode set to Clear to fade the middle more than the edges. But you can also draw a picture you want to use as a reflection. Just imagine a mirror with the same shape, and in that refection you see yourself and everything behind you. How much you want this to reflect by fading with the capacity settings in how much you want this to reflect as much as shine. One thing that will help in a reflection is it only shows the bright areas and never the shadows so the dark areas don't make the shine dark so you will need to use a light layer mode to hide the dark areas.
Drawing bubbles and other shiny things- if it is round you will need to select a round highlight area and use a soft brush to blend more white on the bottom of that. Then unselect that and blur it a little and fade the shine a little. There are other things you need besides reflection and that would show it is shiny if the dark areas shine something a little lighter, so you can do the same with the shadow area as you did with the highlight, but using a color reflected in the shadows, and again even if this will be on the bottom it will have more color on the bottom.
You can learn more by looking up glassy buttons tutorials on google. I am just going through all of this from memory so just in case something don't look right I may have forgotten something. The best part of drawing a shine is if the light comes from a window, it will have the shape of the window in it. If it is an overhead florescent rectangle set of lights there might be a few lights in the reflection. Anything you do with bring that reality to the viewer and you. So get something shiny and look at how that looks. Study it, and because you are fascinated it it, there are somethings you might have a tough time seeing. Once you include ever element you might not be as fascinated with it. The amazement of mystery was revealed, but you can use that to amaze other with your art.