Because the violet is complementary to the yellow, the glow of the center is enhanced. This was J.M.W. Turner's method. Avoid hard edges and let the colors blend to enhance the effect of the glow. Unsized paper absorbs paint like a blotter, preventing manipulation of the paint. Sizing allows paint to flow over the surface, improving washes and permitting the lifting of some colors.
Manufacturers use different methods and materials for sizing watercolor papers.
Internal sizing is mixed with the paper pulp and external or tub sizing is applied to the finished sheet of paper. Some papers have both. A heavily sized paper may resist paint a little too much, so sometimes I lightly sponge a sheet of paper before I begin to paint to soften the sizing. If you stretch your paper, you probably won't have this problem. A 4x5 transparency is a film positive just like a 35mm slide, only larger.