why are feral dogs (ones that have been feral for generations ....like dingos for example) usualy smaller and a different colour than wolves?

Physical differences between related species, such as domestic dogs and wolves, are dependent on two factors: One, the environment in which they evolve heavily influences many factors, such as pelt colour and size. Two, the genetic starting point of the population, and any subsequent gene flow into it, is critical in determining the evolutionary trajectory of a given population.

To put it another way, wolves are big and grey because their ancestors encountered conditions that selected for those traits, and they had the required genetic raw material to allow those selections to happen.

There are "naturally" occurring populations of canines which are smaller or different colours than wolves, such as hyena or the African hunting dog. In contrast, so-called "feral" dogs (I wouldn't really call dingos feral, they are an integrated part of the Australian ecosystem and have been for thousands of years) have been heavily altered by human breeding, with a huge array of phenotypes. Mostly, domestic dogs are smaller than wolves, and colours vary widely, so any feral populations that are founded from domestic populations would tend to be smaller and more variously coloured than your average pack of wolves.

Over time, in natural populations, these differences tend to be evened out through selection, but this process would be inhibited in feral populations due to gene flow between domestic and feral populations introducing a steady stream of variation.