What do you want to compare? The colour? Well, yes.
And maybe the size.
Ultimately it is the composition of the volcanic ash that is of real importance to the question of flying in the presence of ash clouds. To be more specific it is the amount of silica in the ash that might ingested and remelted in an aircraft engine that is of key significance.
The colour of the plume gives some clues to this key question but is of itself not very revealing.
Volcanic Ash, What it can do and how to minimize damage
NOAA Economics of Volcanic Ash & Dust Storm Data and Products | Commerce & Transportation | Extreme Events