SHA3-255, one bit less I need a SHA3-255 or 511. What if I simply truncate a standard SHA3-256 or 512? Apart from the doubled probability of hash collision, are there any other things I should be aware of? I could also truncate one byte instead of one bit, if useful.

What I need is being able to store something different than the hash, in the same 32 bytes, or 64 bytes, so I need to sacrifice one bit to mark when the bytes represent a hash or something else.

Alternatively I could say that if the first byte, not bit, is 0xff, then the remaining ones represent something else. This should reduce the probability of hash collision, but what I would have a probability of 1/256 that a hash starts with 0xff, generating ambiguity in my encoding. I could say that if the for 4 bytes are 0xffffffff, then I would end up with probability of generating an ambiguous encoding of 1/2^32, but I would prefer having a well-defined encoding under any circumstances.

Any well known approach I'm not aware of?
With all well-regarded hash functions, the bits of the hash all have equal worth: as far as anyone knows (unless they aren't telling), the bits are not correlated. If you take $k$ bits of an $n$-bit hash, you get a $k$-bit hash function. Truncating SHA-256 to 255 bits gives you a hash that's almost as good as SHA-256: it has $2^{255}$ strength against preimage attacks and $2^{127.5}$ strength against collision attacks.

There are precedents for taking certain bits of a hash. SHA-224 and SHA-384 are obtained with calculations that are essentially the same as SHA-256 and SHA-512 respectively, just with different initial values (which shouldn't matter to the strength of the algorithm) and with the output truncated to the smaller size. Another precedent is that UUID can be constructed from 122 bits of MD5 or SHA-1 hashes (out of 128 or 160 respectively).

For SHA3, there's a cleaner construction. Instead of taking SHA3-256 and cutting off one bit, take the SHAKE256 255-bit or 248-bit output. SHAKE256 is exactly the same as SHA3-256 except for two bits near the beginning of the calculation, so it has the same security properties, but it's explicitly designed to have variable-length output. You can even use SHAKE128 instead of SHAKE256, which is slightly cheaper to compute with no meaningful security loss.

It's unlikely that there's an actual security problem with truncating a hash, but using SHAKE gives you greater confidence that nothing will go wrong. Even better, to avoid domain collisions (where two parts of the system calculate the hash of the same string for different reasons), calculate the cSHAKE output with a unique string as the customization string.