Welcome to the forum!
Pepper stems change color as the surface matures and hardens, internal growth can appear when the stem is still translucent and different varieties can display differing conditions of this. Bottom line, your seedling looks normal....and healthy I might add!
As to your watering question this is very subjective and dependent on what your potting media is along with air temp & humidity when outside. Over watering is probably the #1 reason for leaf color issues and seedling growth questions. Bottom line to my advice is soak container and note weight, wait for plant to droop then note weight......water when close to this weight.
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