Abbreviations: Texture and Color - TC; Background - BG; Plazma Burst 2 - PB2.
Definitions: Texture - combination of differently colored pixels that form a picture (in PB2 it's background or decoration basic model that can be found in level editor workshop); Color - visual perception of light reflection (red, green, blue).
Map example of using this method: curiosity-pb3demo
This instruction applies to Plazma Burst 3 too. This instruction applies to use of decorations too. This instruction may apply to other games level editors too.
Send this instruction to all your friends map makers if you want to improve their skills and help them create better maps.
////////////////////////////////////INSTRUCTION_BELOW////////////////////////////////////////
Step 1 - Understand the main principle. Best PB2 BG TC is natural looking TC. Natural looking TC is TC that our eyes are biologically used to see everyday. TCs that our eyes are biologically used to see everyday are real life TCs. If you want to make a good BG TC in your map it should have TC similar to real life TC (similar but not the same).
Step 2 - Google images of things you're trying to build. Imagine you're making a bunker map. Bunker map is made of different parts. Let's say you're making a kitchen part. Google "bunker kitchen" and browse until you find something you would want to see in your map. In the image look at the TC of the wall. Look carefully at three things of it: color, shade of color, texture of wall. Learn and remember them.
Step 3 - Recreate the google image in level editor. To do that you will need to pick the right background/decoration model in level editor's custom decoration workshop and, after that, color it with HEX color codes.
Before you begin - remember that your BG recreation should not be 100% real life looking, too bright or too dark, because otherwise it won't match TCs of PB2 walls, characters, guns and other elements (which are not realistically looking and have their own color shades, and yes, everything in the world has its own TC). The golden rule is that all elements of the picture (backgrounds, walls, decorations, characters, guns, ...) should match each other which means that TC of one element should match TC of other element. If elements look too differently from each other - picture will look bad and unnatural in the end. Imagine Minecraft Steve in some Call of Duty (COD) game - it's an example of how TCs (and also forms) don't match each other: pixel Steve is standing in front of a detailed COD tank - looks weird because their TCs don't match each other visually. But on the other hand - pixel Steve would look good standing in front of a Minecraft forest - because their TCs match each other. Same with some COD soldier standing in front of a COD tank - TCs match.
To find the right color and shade of color use websites that let you pick HTML color codes (google "HTML color codes". Example: https://htmlcolorcodes.com/). To find the right background or decoration model (texture) you will need to dig really deep in the custom decorations workshop and pick the right one by trial and error. Try to pick the texture that looks most natural and close to the one you saw in a google image. If necessary change texture (decor) visibility by triggers or cover one decor over the other until you reach perfection. If wall is made of complex elements like wallpaper+wood (like in curiosity-pb3demo) then pick right TC for each of these two elements and if when you put them together they match each other visually - that's a win. If they don't match - change TC of the element that you feel look worse than the other. Change it until your eye says "Yes" and things match.
It may take a lot of time in the beginning to change BGs and test them over and over again, but when you do it a couple of times you will get used to it and it will become simple. Once it becomes easy for you - you can start experimenting with TCs yourself without using Google - you will be able to come up with right colors, shades and models yourself, no help needed.