Pop the reservoir cap and find dark brown or black brake fluid instead of clear amber? That color change is a clear signal. Here is what black brake fluid means.
It is old and contaminated
Fresh brake fluid is clear to light amber. Over years it absorbs moisture and picks up tiny rubber and metal particles from seals and lines, turning it dark; see contaminated fluid signs. Black fluid is simply worn-out, dirty fluid.
Why it matters
Old dark fluid has absorbed water, which lowers its boiling point (raising fade risk) and corrodes the ABS unit, calipers, and lines from the inside; see fluid change interval. The color is a visual cue that protection is gone.
You cannot fix it by topping off
Adding fresh fluid to a black reservoir barely helps - the bad fluid is throughout the system. A full flush that replaces essentially all the fluid is the fix; see brake fluid flush guide.
What to do
Flush the system until clean fluid runs from every bleeder, and keep to a time-based schedule so it never gets this dark again. Keep the whole system healthy with quality pads and hardware.