Do you know the flock and individual number which was on the lost tag? If so, you can phone Ketchums and they will replace the original with an identical tag, any colour, and I think it's free except for postage.
If you don't know the number they will send you a red replacement tag with your flock number on, so have that handy when you phone.
The whole process is automated so they know what individual numbers to allocate, and they are very helpful to newbies, so don't worry.
Whether it is EID or not depends i think on where you are - in Scotland slaughter tags are electronic, but I think that in England for slaughter below 12 months they are not. But someone on here from England will know, or Ketchums themselves will keep you right.