A few thoughts - the Alias is smaller in diameter than the Axis. That should make them different enough. Those who love the Tangent will most likely buy the Alias. I'll certainly give the Envy a try.
Also, fans of MVP seem to collect all the different flavors. Increasing the color combinations could increase sales among current customers while appealing to a broader market like: females, youths, disc customizers.
If indeed discs with multicolored over-molds prove to be difficult to produce with consistency or a bold move in the product direction flops it could be safer to have a distinct brand to protect the core company.