Does anyone know...

if 5700 and 6700 brake lever blades/paddles/whatever you call them are interchangeable?

I want to run silver levers on my ultegra brifters.

Without looking at both levers at the same time it’s difficult to know if it would be possible to change lever blades although as a general rule I would suspect that it would not be possible.

I know this answer probably does not help you much although I figured that it was a reasonable question to ask.

Shimano Australia has an office in Sydney and they often deal with technical enquiries from both trade and public and I think that they would probably be able to give you the best answer to your question.