I’m growing these cacti for about 9months i just changed their soil and pot, while i was picking up the little babies is realised that some of them changed color to red (meaning too much sun i guess) but others are now white ish. I used to put them under direct sun light which i realised it was too strong for the moment. I water them each two weeks. Can it be a watering problem or because i didnt changed the soil earlier? If anyone have an idea on how can they get back their perfect green color it will be really nice. The cacti are san pedro’s. Thanks
Red in cactus, particularly seedlings is related to stress. It may be sun stress, it may be water stress, nutrient, temperature, etc, it's stress.
You say they aren't under direct sun, what sort of sun do they get, for how long?
You changed the soil after 9 months, what was the reason/motivation for that? Unless there is an issue with the soil, it should not have to be changed more than every few years.
Have you ever fertilized these plants?
After nine months they should be much larger.
At what temperature are these plants kept, do they get any heat from below?
My advice; put them where they can get good sun (not all day, but for a few hours at least. Water when they are almost dry, not on a schedule. Fertilize! Use a balanced fertilizer (7-7-7, for example) and use at half strength during the warm growing season. If the soil is cold, consider a seedling mat to warm from the bottom (gently). Or a warmer, sunlit location.
The red color is caused by high light levels, but not necessarily for the reason you think it is.
The red is always there, but if the plant is grown in lower light levels it produces more green chlorophyll to try to keep its metabolism going fast, and the extra green masks the "natural" red color. In their natural habitat (high altitude near the equator) the balance of red and green tends to make them look a greenish gray color.
So the red is nothing to worry about in itself, so long as the plants are not literally getting fried by too much sun. They grow in fairly high rainfall areas, so they need more water than "desert" cacti to grow well and quickly. Watering every week rather than every two weeks won't hurt them, provided you don't overwater.
The areoles (where the spines grow from) are a whitish color. I wouldn't worry about the white with very small plants like the ones in your photo - they probably haven't got to a "mature" balance of the various parts of the plant yet.