There's an effect on the brightness. Primarily because everything the pickup 'sees'; the values of the pots, tone capacitors, the capacitance of the guitar's wiring and also the guitar
cable, all affect the highs slightly, in a way that's more noticeable to the player than to a listener. Because it mostly affects the initial attack transients, right as we pick the string, the difference is less noticeable after a note has rung for a bit.

The values of the pots and capacitors will all affect the tone of your guitar, changing these values will allow you to make small adjustments to the tone of most guitars.The most common standard values are typically 500K pots, and .02uf caps for Gibsons, and 250Kpots and .05uf caps for Fenders. These values have been changed over the years, but the basic effect is the same for all (passive) pickups.

1. LARGER value pots will shift the tone to a slightly brighter range, and may increase noise slightly. This is the reason many Fender guitars use the lower 250K value, to provide a slightly warmer tone and lower noise from their bright single-coil pickups.

2. SMALLER value tone caps sound brighter, larger caps can increase the mid-range response of the pickups. This can affect your sound even when the tone control is turned on '10'. The radical shift in tone on a Strat bridge pickup demonstrates this effect. The bridge pickup has the smallest value tone cap (NONE) and produces the brightest tone. This effect is more noticeable on high-impedence single coils than on humbuckers.

The common complaints about changing from the standard values are that high-value pots can have an abrupt taper and dull tone when the volume is turned down, and that larger value tone caps can cut too much volume. Both of these affects can be easily controlled, which will allow you to actually improve the range and effect of both the the volume and tone controls...see the faq regarding capacitor values/mods.