What to paint on.

I usually paint on either
canvas covered panel,
primed mdf
or stretched white cotton canvas.

Of the three I prefer the canvas covered panel. I like a firm surface and the weave is close on the canvas so it's quite a gentle support and allows for detail if I feel so inclined.

But last week I bought some linen canvases mainly because Great Art, whose catalogue I take to bed to read, has a deal on. So I  bought 4 of 30 x 30 cms.

I'm now wondering, do I paint a different coloured ground, or do I just pitch in?