I don’t buy CD-R blanks very often, since 50 lasts me quite a long time. Last time I bought them, they were colored on the bottom (green for the ones I had) and you could see how much was burned by looking at the bottom.
I just bought a new spindle of TDK’s, and they’re silver on the bottom, just like a normal CD. And you can’t see a line where it was burned to. If you bought a label on it, you could sell it as a normal audio CD.
What happened?
Just out of curiosity, is it mentioned on the package that they’re silver on the bottom? I almost wonder it they’re like that for the reason you mentioned. So people can put a label on them and sell them as an original.
Different companies use different color dyes. You can find green, gold, blue, silver, etc.
Note that some dyes work better with some recorders and players than others so sometimes you have to avoid certain ones. But keep in mind that same color does not mean same dye.
And if you hold a silver CDR up to the light just right you can tell how much has been written.
Nope, just “Writable surface”, which doesn’t really make much sense.
“Writable surface” - is that the side you can write on with a Sharpie or the side the laser writes on? 
Either way, commercially-produced CDs don’t have labels placed on them, so it would be obvious that someone’s selling copies. The commercial stuff is (I think) screen-printed. A fairly close second is the blank CDs that can have labels printed directly on them with specialized inkjet printers.
And many more. My favorite, in fact, is the Maxell Black. Now there is a CDR that makes it impossible to tell where the burning stopped by visual inspection…
HOw can you tell by looking at a CD how much has been burned? 
theres usually a visual diffrence in the coler\visual texture in the written/unwritten portions
Is is true that the green and blue dyes are cheap in quailty compared to the silvers?
Remember too that CDs start from the middle and work out, so the written bit is the inner circle.
A friend who works as a lawyer in the electronics industry told me that the reason there are so many different dyes is that they are all patented to one company or another; essentially, each new company wanting to make CDRs has to develop a dye that is different in composition from all the others, but sufficiently similar in properties that it still works.
A big problem with early CDRs was general lack of reflectance - the writable surface was dark - it simply absorbed too much of the light used trying to read it and the machine treated it as unreadable. More modern CD players have generally overcome this, but while they were moving in a direction of being able to read a wider diversity of CDs, the dye technology was also moving in a direction of being more similar to pressed CDs.
If you want the best CD-r’s buy taiyo yudken(sp?) dye cd-r’s. They are near universally considered to be the best in write speed, durability, compatibility and longevity. They sell under all sorts of brands. I most commonly see them used in Fuji brand cdrs. However even Fuji has started to use other dyes. To be sure look for a “made in japan” somewhere on the packaging. I believe every other dye company is taiwanese.
looking at my discs right now they are a metallic bluegreen color(more on the green side).
Like ftg says, you can make out howmuch of the CD is written, if you hold it upto the light just right. Except if you write onto the entire CD or if you write very little, then it’s rather difficult to tell.