Plastacine Writing Tray

I remember in primary school having a plastacine tray that we practiced writing in, and I tried to recreate one today for Bubble to use but it didn't turn out so good. If anyone knows the 'right' way to make these can you please let me know as I'd love to make one.
The one we had at school had brightly coloured plastacine underneath and a layer of black plastacine over the top, we would then use popsticks to practice our letters and the colours would show through the black (kind of like these black crayon scratch pictures but with plastacine). Then you just smoothed over the black plastacine and started again.
Bubble still really enjoyed the tray I made for her and played with it quite happily, it worked in that it showed up her marks and lines when she drew but the colours underneath didn't show through at all (maybe the black layer was too thick?).

WE USED:

* shallow tray/dish
* plastacine
* mini skewer or chopstick

It was a disaster to begin with lol, I ran out of black so had to use the next darkest colour to finish the top layer - brown :-D

Bubble's drawings still showed up well, just not in rainbow like I was expecting!

Eventually we flipped our plastacine over and drew on the colourful side as well, which worked pretty nicely too.