Up to 10% but generally there is little difference between 5 and 10%. Powdered dyes are far more potent as an integral dye than liquid versions. We recommend that you carry out your own tests with dyes and record and label all your mixes.
Yes you can add dyes to add colour to your concrete. We sell a selection of dyes on our website. We advise that you purchase a small quantity of dye and do some tests because colours on screen rarely reflect the colour in real life. Here is a link to the dyes we sell.