steezz is right, it looks like you've over watered and I don't see any drain holes in the bottom of the cup. Cut a couple of drain holes in the cups and let them dry a bit.Overwatering will be easy for you to do since you don't have much drainage on the soil you're using from my looks of it.
try to water with a mister for a week, thats what I did worked fine.
Well if you have a cool/vented hood, you should be able to keep the 600w hps about 18-24 from the top of the plants, if you do not have a air cooled hood, then it might be hard as you want temps to be no higher than 85, but idealy about 75 degress. Seedling like a very humid enviroment (80%+RH), so I like to use a the dome untill they are on thier 2nd to 3rd node, at that point I transpant them into the big pots.(no more dome) I keep my 400w MH at 18-20" above the canopy. A good rule of thumb is, if the heat from the lamp is hot on the back of your hand at canopy level, then its to hot for your plants. I use a mix of Fox Farms ocean forest soil, light warrior seed starter soil, perlite, and lime (lime added at 1.5cups per qubic sqft of soil) so depending on what soil you use, will also depend on when nutes should be started.....some soil can be hotter (has more nutes in it) In general I dont give them any nutes for the first two to three weeks of growth. Start nutes at 1/2 the general feeding recomended amounts and see how your plants react, once you know how to read your plants, feeding and watering will become easy. Hope this helps, and good luck on your grow.i have a 600 hps hand i do have a humidity dome,,"incubator" i just took the lid off to take the pic.. how close should i keep the babys from my 600?? i dont want to burn them..also what age should i start giving them nuets?? i am going to go with the 18/6... thanks bro