The designer of the Do-it molds (formally Caney Creek) Jason Barnes used to post that the molds are designed for medium. That's all I use.
I agree on buying the colors that you currently buy the most baits in and have the most confidence in. For me that's black, silver smoke, watermelon brown and chartreuse . The glitter that I use the most (by far) is black, blue, red and silver. I also like rainbow glitter for when I can't make up my mind! (it's great is clear or very light silver).
I would also suggest white..but it's a tough color to get right...so save that for a few months from now.
Make sure you get a thermometer

(I always say that!) and get your plastic to 350 so it fully changes state (or you will get cloudy or sticky baits) but don't let it get too hot (380) or it will start to "burn" and you will have problems with yellowing...so temperature is important.
Read and follow ALL of the postings in safety! It's a fun, rewarding hobby ....but do it safely!
Good luck...and remember we all have done it "wrong" ...more than once....so read up..and ask questions. You will be amazed how good your baits will turn out.
Andy
Andy