Well putting it in English was painful >,> Hopefully I still make sense in it? Anyway this is probably going to be a series of quick tips/guides covering a range of topics relevant to unity and accent, precisely composition, statics/dynamics, flow, etc.
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You can read about color theory here: [link] More tutorials could be found here [link] No stealing, I kid you not
It makes perfect sense, don't worry about your english. And I totally agree whit you in every point, these things are basic in any composition; but many people don't know them so it's great that you can take a little of your time to teach this.
One solution I've used, and I realize now that I've gotten away from it, is to shade one element to more closely match the other. For instance, with the model in the red dress in front of the brown background, I might have duplicated the brown layer, put the new layer in front of the model layer, run the Photoshop Average filter (Blur>Average), mask the layer to match the contours of the model, then adjust blending mode and opacity to get the colors right.