Yes, the 4 pounds of Trichlor adding 22 ppm FC would make a Free Chlorine test register > 5 ppm so would be very dark (yellow/orange for OTO test; DPD test would bleach out). So yes, that was the cause. The chlorine level will continue to drop over time -- especially if the water is exposed to the UV rays of sunlight.
The part about replacing reagents is valid, but every year is overkill for most of the reagents. Some will last a very long time -- the acid and base demand reagents are like that. Next are the indicator dyes. The ones that last the least are the FAS/DPD titration drops since they are sensitive to light -- that is, the R-0871. If you want a good bargain and a better combination of reagent sizes, most of which provide 36% more volume so more tests than the K-2006, then check out the TF100 here
. It will also test the CYA level down to 20 ppm. The pH test is different than the K-2006 -- it shows a wider range with less granularity (it shows 6.8, 7.2, 7.5, 7.8, 8.2 vs. the K-2006 that shows 7.0, 7.2, 7.4, 7.6, 7.8, 8.0). I started with the K-2006, but now have a TF100. The TF100 uses Taylor reagents so it's really just a more intelligently packaged Taylor K-2006 with more logical sizes for the reagents. I show the contents of the TF100 in this thread
You can prevent getting algae in the future by maintaining an absolute minimum Free Chorine (FC) level that is at least 7.5% of the Cyanuric Acid (CYA) level. A better target is an FC level that is 11.5% of the CYA level (roughly 10% of the CYA level is fine). So it's important to not only use stabilized chlorine (Trichlor or Dichlor) and instead to use unstabilized chlorine (unscented bleach or chlorinating liquid or Cal-Hypo) though with Cal-Hypo you need to watch the Calcium Hardness (CH) levels.
For every 1 ppm FC added by Trichlor, it also adds 0.6 ppm to CYA. For every 1 ppm FC added by Dichlor, it also adds 0.9 ppm to CYA. For every 1 ppm FC added by Cal-Hypo, it also adds 0.7 ppm to CH. The problem is that whereas the FC will drop over time (i.e. get used up), the CYA and CH will not as these only decline via splash-out or dilution (CYA can slowly degrade in other ways, usually over a winter when a pool is "let go" without chlorine).