Powders need a certain amount of pressure to burn cleanly. As you load cartridges to reduced charges and therefore reduced pressures, you will get increased fouling. As a general rule, most powders will burn fairly clean at the upper end of the charge range and burn dirty at reduced loads. And within that general rule, some burn more cleanly than others, of course.
I shoot .45ACP for bullseye. The .45ACP is a low pressure cartridge to begin with, and loading it down somewhat for target shooting makes fouling even more of an issue. Given that, I (and lots of American .45ACP shooters, which is where I learned of it in the first place) like Hodgdon Clays (not Universal or Universal Clays, just plain Clays). It can be loaded quite light and still produce remarkably little fouling. A common load is 3.7-3.8 gr under a 200gr LSWC.
A word of caution, however: it is able to do this because it builds pressure more quickly than other powders. This means that loads near the top of the scale can develop dangerous pressures very quickly, too.