It's impossible to tell by color. Around here most of the dope is beige/off white to light brown. Obviously it varies, so the color doesn't mean much. When heroin is made they may repeat the final step until it's white and a little more pure, but if they don't it would be a little darker but still close to pure. So right off the bat it varies in color, even though the purity would only vary a tiny bit. The impurities make it a little brown, but it could be 94% pure like that, and the final step that gets rid of the impurities will make it whiter, but just up to like 97% so it doesn't change much. This is all during the production before it gets sent to various destinations to be sold.
Then once it hits the street and is cut, it can either be cut with a lighter cut or a darker cut. They can use a tiny bit of a dark cut which would make it really strong and dark, or they can use a lot of white cut, making it lower quality and white. Then there's every combination of those things, starting with varying shades of almost pure heroin sent here from South America, then being cut with varying shades of cuts.
Even if a certain color dope is usually good, there is no telling until you try it. IME white dope is either really good or really bad, with no in between. That doesn't mean that I won't come across some average white dope at some point, as I'm sure many people have.