In normal colour theory, they're considered monochromatic. So are they different colours or considered as one?

I believe each shade is a different colo(u)r when it comes to the game jam's qualifications.

In this jam each color that has it's own "color code" is considered a separate color. However, there is an exception to this in the rules. Rule: "Gradients are allowed but no more than 3 objects/sprites/backgrounds in total with a gradient and the two main colors from the gradient count towards the 10 color max.". So basically you can still do shading with gradients but you have to follow that rule. Btw you could also do shading using dithering.

Hmm okay, thanks!

to clarify please.

this isn't allowed because it has 4 colors.

but this is allowed because it has 2 main colors, but one of them has a gradient of light and dark?