There were some very dark patches of deep blue on the cloud, which I found interesting as I had never seen it before. The deep "unnatural" colours are the result of post-processing. I'm a fan of greater saturation, just my personal preference. There's no point using a tripod in broad daylight unless your arms are made of jelly, this again is in my personal opinion. I shot the image at ISO 250, and personally I think it's fairly clean. The "graininess" is likely as a result of compression.