A test kit will go a long ways towards helping determine what if anything needs to be added, nitrate phosphate potassium (NPK ) are the macro nutrients plants need to thrive, test kits are available for phosphate and nitrate, though a hobbyist level test for potassium isn't available, though the general consensus seems to be it's not needed.
After that, if you decide based on additional reading that you need to dose nutrients, you can buy dry fertilizers (or wet, but dry are WAY cheaper for the volume you'd want), there are lots of variations to the chemical structure's of the ferts, depending on how you want to fine tune things (if you get so far) nitrate is generally kno3, phosphate is generally kh2po4, and potassium (if you really really actually need it) is k2so4, though the kno3 and kh2po4 both add some potassium already. Some of the other commonly dosed minerals are calcium, magnesium, and iron, with trace minerals rounding out the bunch (rare earth metals, and the like in very very small amounts, and again usually not needed unless you have a specific reason). These dry ferts are prefered over say a commercially available hydroponic mix, because you can fine tune what ratios you want for your plants based on observed usage from testing
As to the lightening of the green in the leaves of your plants, some of that may be attributed to the intensity of the light they are now receiving vs how much they would get if grown in full sun outside. It can also be a nutrient deficiency (usually nitrogen) though would probably require pictures and someone a bit more practiced in identifying deficiencies to chime in.
After that, if you decide based on additional reading that you need to dose nutrients, you can buy dry fertilizers (or wet, but dry are WAY cheaper for the volume you'd want), there are lots of variations to the chemical structure's of the ferts, depending on how you want to fine tune things (if you get so far) nitrate is generally kno3, phosphate is generally kh2po4, and potassium (if you really really actually need it) is k2so4, though the kno3 and kh2po4 both add some potassium already. Some of the other commonly dosed minerals are calcium, magnesium, and iron, with trace minerals rounding out the bunch (rare earth metals, and the like in very very small amounts, and again usually not needed unless you have a specific reason). These dry ferts are prefered over say a commercially available hydroponic mix, because you can fine tune what ratios you want for your plants based on observed usage from testing
As to the lightening of the green in the leaves of your plants, some of that may be attributed to the intensity of the light they are now receiving vs how much they would get if grown in full sun outside. It can also be a nutrient deficiency (usually nitrogen) though would probably require pictures and someone a bit more practiced in identifying deficiencies to chime in.
