I would start with a uniform coat of gray primer.
I would take a whipped butter cupful of light green, rust brown, and black, and alternatively dip a synthetic 'natural style' sponge in one color at a time and dab it at random over the gray.
All these colors are available as vinyl latex exterior wall paint, and that's what I'd use. Flats work fine, satin's OK, gloss, maybe not.
Greg
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"Government's view of the economy could be summed up in a few short phrases: If it moves, tax it.. If it keeps moving, regulate it. And if it stops moving, subsidize it." - Ronald Reagan