First things first, you want an avocado that is soft to touch, but consistent all over. If you give the avocado a slight squeeze and it is extremely soft in some spots, but hard in others, this just means the avocado is bruised. When it is soft at every feel, it is perfectly ripe.
Now don't be fooled by the color. Most people will intuitively think that the lighter green avocado is not yet ripe, whereas the darker purplish colored avocado has further ripened. This is not necessarily the case, but rather relates to the type of avocado. The all-so-popular Haas avocado has the purple hue, whereas many of the other varieties are green.