I have not used the sponge techniques, but I’ve painted a few guns. I’ve only used Krylon, they make a decent assortment of colors, I can’t speak about other brands. I do not try to duplicate any specific pattern, because I, too suck at arts & crafts, so I’m just looking to break up the outline with natural colors. 3 colors is a good start, a tan, a brown, and a green. More colors/shades make it better, IMHO. I’ll try to get photos to download of my 870, it’s the only gun I still have that I’ve painted.
I’ll tell you what I do, but this is all stuff I got from articles & the internet. Don’t worry about screwing up, if you really do, you can paint over everything & start again.
Start with a light color base layer like you have. Pick colors that match the environment you’ll be in, or what you think looks cool, it’s your gun. I use stencils cut from cardboard, & mesh. The carboard stencils do not lay flat, which gives a “blurry” edge to the shapes, which adds to the camo effect. I cut a lot of stencils, some with tiger stripe style lines, some with GI woodland style shapes, some with starburst shapes, anything will work. Variety is important. Use the stencils to put color patches on the base layer. You can overlap the stencilling for a “busier” camo.
The mesh is used in the final steps to take away sharp lines & blend colors. Use it anywhere on the gun to add color & minimize lines & large solid color patches.
A “mist coat” can be applied at the very end by holding the can about 12" away from the gun, this again reduces sharp lines. It will produce a “spotty” effect, but the spots will be very tiny. I prefer a lighter color for the mist coat, as I usually end up with more dark color that I want. If your camo is looking too light, you could use a darker color to make it more to your liking.
You have a light color base coat, which is good. It’s easier to go with a dark color on top of a light color, not so much the opposite. Keep that in mind when applying colors.