Any home goods store here (USA) has seats in many materials and colors. My parents used to use cushioned seats with vinyl on top, hard plastic on the bottom. My dad's bathroom had blue and mom's had magenta.
Had a mauve colored one in the house i grew up in. Apparently it was the thing to have in late 80s early 90s if you were building a house in the US. Matching colored cast iron bath tub as well. Rather ugly in retrospect, but thats fashion for ya