A black olive doesn’t have more oil than a yellowish one. It just has less water.
The olive’s year goes roughly like this. May to July: after fruit set, the fruit builds its cells (90% of the pulp cells form in the first eight weeks), and final size depends mostly on how many cells there are. That phase needs lots of water, which is why a rainy June matters for the autumn oil. July to September/October: the stone hardens and the olive starts making oil, slowly at first, then fast in August and September. October-November: the skin turns from green to yellowish, violet, black. By around the end of that colour change, all the oil is already there.
After that, the fruit just dries out, so the oil weighs more in proportion. Rain rehydrates it and the yield drops for a while, with exactly the same oil inside. Riper olives also give oil with fewer polyphenols.
Yield (rendimiento) is oil as a percentage of olive weight: 20-25% is normal, so about 4-5 kg of olives per kilo of oil. Labs also measure it on dry weight, the number water can’t fool.