Different Advantages come with each technique.
If you separate the grey from colours, you can toggle either one separately. If you wanted to change the shape of the watercolour, you only need to affect the grey layer and then edit the colour later. The different values of colour are already chosen for you by the different greys beneath, so there’s no guess work involved. One disadvantage is that the colours won’t always come out the way you expect unless you toggle the layer properties.
Advantages that come with colouring all on one layer is that the colours may come out brighter because you’ll be picking them yourselves. You can achieve the same effect with one layer and without having to toggle any layer properties. The disadvantages include needing to pick colours by eye to get the different values, and that may be more time consuming than if you were to only select greys. Changing colours will require separate layers in order to preserve the values that has already been established.
I hope this has been helpful and informative to you! Happy arting, Keep making the world beautiful :D!
