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Colonies of Gram-positive skin bacteria growing on a Mannitol Salts Agar (MSA) plate. These photos demonstrate the power of selective media. I swabbed the skin on my forearm and then inoculated a pair of plates with that same swab. The first (red and yellow) plate was MSA, which selects for Gram-positive bacteria. The second (burgundy) plate was Eosin Methylene Blue (EMB) which selects for Gram-negative bacteria.
The skin of your arms is dry and a bit salty -- a tough environment. The membrane that makes up the outer layer of the Gram-negative cell wall is sensitive to desiccation, so many Gram-negatives can't grow in dry environments. Gram-positives have a thicker cell wall with no outer membrane, and tend to be desiccation tolerant.
The majority of human skin flora is Gram-positive, as these plates show. The MSA plate, which encourages salt-tolerant, Gram-positive bacteria, grew many colonies. The EMB plate, which encourages only Gram-negatives, grew few.
Microbiologists use selective media to grow those species they want to study and discourage the others.

Collected from Loch Norse, the NKU campus "lake", by General Botany students using a diatom net. First photo at 400x magnification, others at 100x. Pond is normally treated with aquashade to reduce algae growth, but hadn't been done recently. Water level was very low.
Note heterocysts (round, clear cells) for nitrogen fixation and aerotopes (dark circles in the cells) for buoyancy. Last photo includes a couple of grains of pine pollen, I think. (Looks like Mickey Mouse heads).