Hi Jodie, these were done in a few hours each. Watercolour is transparent - almost like working out the bottom layer first and this will show through as you paint the next layer, so you have to think in advance. Using white watercolour because of it's transparency, one cannot cover the undercoats well so I use white gouache (opaque) for the white feathers and add it to watercolour for the highlights = the light tints on the head and wings.
I love using watercolour at the moment because it dries fast. I am using the artist quality type not the cotman (student quality) as the pigment appears more intense hence paying double the price.
Not sure about poster paint, I used a dark blue poster paint once for a night sky - I was painting an owl in flight. The paper was 25years old and I used it purposely to hide the aged stain. My husband still takes the mickey and tell people that I got £50 out of a piece of mouldy paper! Well, the watercolour plus poster paint piece is over 3years old now, and I see it's owner every Friday and he hasn't complain yet!
You must have a go at using watercolour Jodie or even use Derwent watercolour pencils then run a wet brush over it for watercolour effects, it's fun. Presently, I am playing on the idea of thin washes - would like to interpret see-thru material - got rude subjects on my mind <<, these probably sell better than birds with wings! Lily