En general, dark green to faintly red (which may appear brownish) indicates a good level of light, bright red to pink suggests too much light, and pale green to yellow may indicate too little light but may also be a sign of other issues.
Although there is a great deal of succulent variation, direct sunshine is too strong for most cactus seedlings. Some species become brilliant red until they produce dense spines, while others seem to remain green up until they are irreversibly damaged.
The overall, succulent seedlings will thrive with less light if the cotyledons or hypocotl are turning pink. When they develop some areoles, increase the light levels. Similar to overwatering, it is considerably more dangerous to give too much light now than it is to give too little light initially and increase it later.
