I drink lots of water but my urine is still yellowish.
The body is able to regulate the amount of water it has so that if it is short on water it conserves water by producing more concentrated urine, which will appear dark yellow.
When one takes lots of fluids, one passes more urine, which may then be lighter hence it may be straw or light yellow in colour, though in some cases it may be colourless. So urine being straw-coloured or light yellow means you are taking enough fluids.
Using the colour of urine to know how much water or fluids to take is useful since water loss depends on the surrounding, temperature or humidity apart from the body activity, all which may never be the same always. This therefore means one cannot rely on eight glasses a day with different water loss situations.