Core Latency is divided in three stages; Early Stage, Middle Stage, and Peak Stage. The higher the latency of the Core, the brighter the color is.
Red Core for example; it has a dark red colour in the Early Stage, neutral red colour in the Middle Stage, and bright red colour in the Peak Stage.
It applies to every Core Stage.
Basil was already in the verge of breaking through to the Green Core. Therefore, his Core had a bright yellow color. It was currently shining intensely, and the glass-like, bright yellow sphere had fissures covering it.
This fissure signifies how close Basil was to breaking through to the next stage.
Once the sphere shattered, the Qi contained in it would form a new and better quality Core—Green Core. This was the reason Basil was trying his best to break the sphere.
Of course, "breaking the sphere" can't be taken literally. One could easily break the sphere—the Core—from the outside, as long as one knew the right position.
To understand Core Reforging better:
Imagine you freeze water into a sphere shape. However, the only thing freezes is the other layer. The water that the ice sphere contained is the Qi, and the ice sphere is the Core.
Then, you crack open the ice, take out the water, and freeze them once again into a sphere shaped ice. That is how Core Reforging works.