Three and a half weeks are considered by many to soon to be feeding your plants. Not by much but maybe by a little. Some people say 2 to 3 weeks before feeding and some say 3 to 4 weeks but I like to go at least 4 weeks and sometimes a little longer depending on the strain and how it appears at the time. I like to play it safe.
New growth often is less green/light green/slightly yellowish until in the sun/light and photosynthesis occurs for a while and then they green up but if they were green and turned yellow then you may have a problem. Just give them a few days or so and see if they green up or not.