Question
I am growing cacti from seeds.  can they be put in direct sunlight?  there are conflicting sets of advice on this.  Also, some go from bright green to yellow or brown-red.  What is happening?

Thanks

Answer
In part, it depends on where you live. In the Southwest, for example direct sun will kill tender seedlings.  Even in other parts of the country any direct mid day sun will kill them.

An ideal location for seed starting will be a location in an open area where the rain and the sun can easily water and warm the seeds. Early morning and late afternoon sun is very advantageous and even with that the seedlings need to acclimate, starting with an hour in the AM and gradually increasing,  but too much mid day sun is too much for cactus seedlings. In their early stages cactus plants need less light and most species will need some protection from direct sun. Often times the seeds will hide under the rocky medium or in cracks where they can choose how far out into the sun they may need to grow. One way to avoid too much light for outdoor seedlings is to place them in a location where a tree or a tall object will provide some shade, while leaving open space for morning and afternoon sun to shine directly on the flower pot. So to answer your question I wouldn't put them in direct mid day sun at all and would very slowly acclimate to AM and PM sun as I said.

Wait for seeds to grow before putting them out and after seedlings start be sure they are provided with water on a regular basis.

As for color change, seedlings often look nothing like a mature plant and will go through a series of changes.  Sometimes the color change is related to too much sun as well and the color change is related to stress.  I can't over emphasize how tender these seedlings are and how quickly too much direct sun or too much water will kill them.