Axles can be a tricky go kart tuning adjustment if you aren’t a veteran with chassis dynamics, track grip conditions, and overall general mechanical / lateral grip on how they with with go kart chassis. In the figure directly below, here is a short explanation on what you see.
The bottom-most part of the graphic represents the on track grip level. Unless there are some fairly extreme weather conditions primarily for the majority of your racing year the track grip conditions will be in the Yellow with medium/neutral rubber or grip on the surface. The primary axle to baseline your setup is going to be the Gold (Oro) anywhere you are located. For example, California or Florida it is quite hot the majority of the year and tracks will become more rubbered up as it is run on all year. This scenario may cause the grip level of track surface to be higher. With INCREASED track grip surface, it is recommended to soften the axle allowing for more flex and free up the rear of the kart. The recommended axle choice for this would be closer to the Orange (Arangio) or White (Blanco). The opposite if you live in New Jersey where the climate is much cooler for a longer portion of the year, track surfaces may not reach a very high grip level. With a DECREASED track grip surface, it is recommended to stiffen the axle allowing less flex and helping to plant the rear of the kart to surface. The recommended axle choice for this would be closer to the Green (Verde) or Red (Rosso).