Didn't read all the replies as I am trying to get through several sections:
Hard to answer but a few things I know for sure - too many specific vitamins and your urine gets dark yellow (no doubt you know that - vitamin B complex for example). Some medication will do this to your urine as well - I take it as a sign your kidneys are working harder but you can't assume that every time. If I see it, I read up on the antibiotic or whatever it is.
Yes, darker urine can be a sign of your kidneys not getting enough liquid and working harder to get the compounds out of your system. Easy answer is to increase your intake of water. Glass of water every hour or two and you should see it become much more clearer - if not then it can't hurt to talk to a doc. Darker yellow or even worse dark urine (as in whisky colour go to the doctor) - can be a symptom of multiple issues but don't panic, it probably isn't. I have been pushing urinalysis strips 9 or more (think they go up to 12 and you can download the booklet to understand each item) at a minimum and you can ask any pharmacist for them. I do it once a week on the weekend. It won't give you a creatinine or eGFR number but it measures your protein and checks for infections, etc. Not near as good as blood tests but what docs use to get at a physical before blood tests were so easy to order.
Vit C will not turn your urine yellow unless you are an anomaly, as for the PED's it would depend on the amount and how well your kidneys are working. I often have much more yellow urine in the morning because I slept 6 hours and didn't drink anything but my body is still removing / metabolizing some medications out of the body.
A simple and old school method of checking your kidneys and some docs still do it is to make a fist and reach behind and hit (not hard) up and down from your waist and up and if it hurts, it is a symptom your kidneys are struggling. I am not stating that works for fact, I am going by my last two docs who would do this, ask for a urinalysis and order bloods.
Best of luck. This is not medical advice as I am not a doctor and just my experiences and from reading. I have thought a few times my kidneys are struggling and they aren't.