should be fine... we have 5 seedlings going right now (though two will get cut here soon). ours have a colorful purple on the stems, but seem to be very healthy... it could be the temp, we are trying to keep ours around the low 70s when the lights are off and usually around 77-80 when the lights are on. im no expert though so don't take my word for it. edit: ours are exactly 7 days old as well...started from seed 10 days ago.
well ive been keeping to room temp at 78-82 but i got the idea to put the thermometer right under the lights and it said about 89-92 to i turn my heater down some
yea wen they are that young, purple stems dont mean much, but wen ur later into vegging, and flowering, i think it means that the plant is stressing
its plant stress, your set up needs tweaking. but its not horrible. now if every stem turns dark purple you'r plants prolly wont make it.
your seedlings are fine. purple colors often appear on stems in seedlings as well as later in plant development. nothing to worry about.