The (chocobo) racing silks

I would consider this scraping the bottom of the barrel, but let's run with it for now.

I never cared about chocobo digging, as someone who hasn't even gotten around to getting all the chocographs for Hot and Cold, but anecdotes about the effect of the blue racing silks, which " enhance chocobo digging endurance," indicate that they dramatically increase the total yield above the usual maximum of 100 items per zone. The sky blue silks, which are supposed to "enhance chocobo digging skill," can give the message "Your chocobo appears to have gained valuable knowledge from this discovery" after a successful dig, but it is unclear to me whether any chocobo digging skill-ups come with that. The blue silks seem like the better value for chocobo digging.

As for Hot and Cold itself, the white and black silks give one additional digging attempt and "increase Jackpot chance," respectively. Since hitting the jackpot allegedly can only occur when your chocobo is "especially sharp today," the white ones would seem to be a better value. I myself have observed that you can receive another digging attempt when your chocobo is "especially energetic," so this status and the white silk effect are not mutually exclusive.

The green racing silks are supposed to "enhance chocobo caring ability," which sounds like it could decrease the amount of energy depleted from both care plans and active care. However, chocobo raising is not something I would expect anyone to do indefinitely, so this doesn't sound like a really good value. Besides, you can raise a maxed-out chocobo with minimal expenditure and attention anyway.

The last three racing silks pertain to a chocobo's speed (how fast it goes) and riding time. It seems as though the orange silks would increase riding time by 10 minutes for both rental and personal chocobos, which is not very useful. Rental chocobos last 30 minutes and personal chocobos with maximum endurance are supposed to last up to 45 minutes. (I'm sure it has been shown but it's something easy enough for me to test, so I will just say "supposed to" until I check it out.) If the orange silks can be shown to increase riding time by 10 minutes for both max-END personal chocobos and rentals, then what about the red silks, which have the vague description "extends riding time"?

Finally, the purple racing silks, which I bought sight unseen because I like purple, are supposed to increase the movement speed of your personal chocobo. From FFXIwiki, which provides a convenient chart showing how levels of STR and END for personal chocobos correspond to movement speed and riding time, respectively, max-STR chocobos are only +95% faster than character running speed, compared to rentals, which again run as fast as Flee.

There is at least one account indicating that a max-STR personal chocobo with the purple silks is as fast as a rental chocobo, although I'm not sure how you can really ensure that your chocobos are lined up so that they run exactly parallel. My own extremely informal testing indicates that my personal chocobo with or without the silks covered approximately the same distance in approximately the same amount of time (about 32 seconds) as a rental, accounting for positioning error and 1-second precision. If the purple silks actually work, sure, why not use them. Again, I just got them because they were pretty.

Overall, the silks corresponding to chocobo digging and Hot and Cold seem to be the most economical uses of 150 chocobucks. To the extent that you do chocobo digging, you want to optimize your yield, and for Hot and Cold, you want to minimize failure. The other ones, not so much, but if you like the colors, go for it.