I have noticed that my didiplis diandre has yellowish leaves on the highest leaves and greenish leaves on the lower leaves.

Any significance to this?

that is a good thing. Healthy DD will usually turn yellow then red on the tops closest to the light.

Is this newly planted D. diandra? I've found that most nurseries/retailers will ship D. diandra that has been grown in emergent form, which usually has much darker green, and somewhat broader, more succulent leaves than you will see it produce in submersed form. There is an almost immediate change in the color of new leaf growth once it is placed in your tank. And just like mlfishman says, this is a good thing