This would be a super huge discussion but the short story is that color (master badge) does not affect flex. Master badge is the dye that is added to the plastic granulate before injection to create colored boots. Master badges can produce solid or transparent colors. The absence of master badge produces a totally clear boot.
Plastics therefore by themselves are colorless but some plastics are better suited for highly transparent colors, like polyether whereas some plastics are too milky/cloudy to be used for high transparent colors, like polyester. Both polyester and polyether are plastics in the Polyurethane (PU) family and are considered high end plastics but they each have their pros and cons. Polyether is great for highly transparent and also solid colored boots but its weakness is that it cannot be ultra stiff. Polyester, being cloudy, is better reserved for solid colored boots and ultra stiff boots. So (usually) if a manufacturer wants to have a super stiff (130+) flex, polyester is better but the boot should be solid colored. It is very difficult to have a transparent and super stiff boot. From here it would depend on wall thickness.
Wall thickness (thickness of the plastic) also needs to be taken into consideration for how stiff a boot is and what material is used. Basically, the greater the wall thickness, the softer plastic you can used to create a stiff boot bu it is heavier because of how thick it is. If the wall thickness is less, a stiffer plastic is required to achieve the same result, but it is lighter because it is thinner. A problem arises when you want to have thin wall thickness and a stiff boot and a transparent boot. Usually manufacturers have to forego the transparent aspect and use polyester to achieve the stiff flex they want to have.
So, generally speaking, color does not affect flex but wall thickness and plastic material do and thus certain combinations of wall thickness and plastic material can dictate what colors (solid or transparent) are used. I am by no means saying that this is what FT has done, it's just a general remark.