English Translation
She decided to paint the wall blue.
Why streichen and not malen?
Short answer: because that’s what you use when talking about painting walls.
Long answer:
“Malen” is generally used for artistic painting, while “streichen” refers to “stroking” with the brush (or paint roller) and simply applying paint.
Also, the object of “malen” is what you paint an image of. So if you say “die Wand malen” you’d mean “to draw an image of the wall” (onto paper or whatever).
There is “anmalen”, literally “to paint at”, which could be used, but that is also more about applying an artistic touch to the wall, for example a painting in some area to look at, rather than simply changing the color of the whole wall.
That being said, the craftsperson you hire to paint your wall is called “Maler”, and not “Streicher”. 
“Streicher” would also be confusing, as it usually refers to the strings section of an orchestra - you know, “stroking” the strings with the bow.
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