I do believe most often light levels are the key to less colourful plants under artificial lights. Even in a dark spot in a greenhouse the light levels are very high compared to typical terrarium setups.
Another factor could be the spectral power distribution of the lights you do use. But this isn't a problem as long the plants do grow well. Even under those yello NaHDL lights (High pressure discharge plant lights - don't know the correct translation) the plants do grow very nicely. Of course the colouration is different to plants grown under sunlight.
When a massive decrease in colouration appears slowly over some leafes it may also be due to a lack of food. The plant may be low in nitrogen or also in Iron (Fe) which the plants do get from the soil. But I don't think this is the case for your plants by now.