When I create four random colors, a and b show up as a row of four swatches

But when I try to create a matrix [a;b], figuring four over and under, all I get is a vector row of eight swatches

What am I doing wrong?

a = [RGB(rand(),rand(),rand()) for x in 1:4]

b = [RGB(rand(),rand(),rand()) for x in 1:4]

[a;b]