Why does pregnancy test line colors change throughout the day?

So I just found out I’m pregnant. 4+2. First pregnancy ever, first ever time ever seeing a positive pregnancy test for myself, so a whole lot of firsts. I just got blood drawn and hope to get the results back tomorrow or the day after. And until my pregnancy is confirmed and I get a first scan, I’ve been peeing on sticks like nobody’s business to make sure I’m still pregnant and possibly from disbelief honestly.

What I’ve noticed and have also had several panic attacks from, was the variance in color of the test line throughout the day. I always test in the mornings and usually it’s my strongest test but I’ve had times when it was lighter than the day before and my evening urine would give a darker line and then calm me down. Does anyone know what the darkness in color changes throughout the day? I was always told fmu was the most concentrated and had the highest levels of hcg but From my test I’m learning it’s not always the case. I wish I could be happy with a line is a line but after reading so many stories on losses and the test getting lighter being the first sign, I can’t.