Everything you are told about keeping variegation is wrong.
You probably heard someone say, increase the light, increase the humidity if you want to keep the white part on your variegated plants stay longer. It's actually the opposite. I will tell you why.
High humidity without proper air circulation will cause water condensation on the leaves. Those droplets will melt the variegation part in no time. So if you see your leaves keep melting, reduce the humidity and/or increase the airflow. These two are integrated, so adjust them in relation to each other.
High light will burn leaves. Morning and evening direct light might work for nonvariegated plants but if you see burnt spots, consider pulling the plant a few feet further from the light source.
Dehydration: If you leave the plants too thirsty, they will sacrifice the most useless part first, aka variegation. Make sure to keep your watering consistent.
Also, other factors like imbalanced PH levels in soil, overfertilizing, root rots, and pests are causes of the variegation deterioration.
If you can make sure all of these factors are in check, your plants will actually keep their beautiful variegation until the rest of their life cycle.