Do you add white first or last?
Both, but usually more towards the end of the painting. Highlights are best done last, but an object with detail (like the flower) works better to lay out the shading on the white area first. Acrylic paint is opaque so you can paint white over dark colors. Generally I start with the part of the painting that is visually farthest away and lay out the shadows with fairly wild colors. Clouds get some white in this stage, so there are no set rules and each painting requires different strategies. Some paintings work better by doing the shadows first then building the light areas on top. To make a light color like yellow stand out over a darker color it helps to paint the area white again before adding the color. Both black and white paint flatten colors when mixing paint so I use as little as possible of either. When white is needed I often mix it on the canvas so the brush stokes show.