Any paint can be used for either.
Washes are essentially tinted thinner. Use water based paint over oil and oil over water so the thinning agent won't erode the paint already applied.
Don't flood the wash onto the model. Wipe the excess from your brush and touch the recessed area to allow the paint to flow.
Also, don't get stuck thinking only black can be used here. A darker shade of the primary colour will look more natural.
Don is spot on here. Practice makes less imperfect. Old sprue is great for this. Also, left over sheet styrene is good