Color amarillento en cotiledones

i don't see yellowing in the pictures... shouldn't "need" fertilizer the first week or more. Then, your soil should have a charge in it that should last a while too. if it just sprouted, it'll fill in... if it happened after the fact, ... ?? have you fertilized? if so stop doing that until you see a need... take not of howlong before that occurs and you can start fertilizing a bit sooner next time. but, shouldn't have any nutrient caused issues this early, normally. removing shells happens sometimes.. usually a function of not planting deep enough or not tamping down the substrate quite enough... smal adjustments can avoid it in future.. it becomes memorize SOP before long. sometimes a little thin layer remains behind you have to carefully slide it off the coty's too. (when you have to remove a shell) Easiest to use a toothpick or soemthing then catch it at split of leaves and gently slide it off. (thin transparaent 'skin' can be left behind at times)