My ask test strip shows red, not purple for leukocytes but negative for nitrite. what does that mean?

Nitrite can be (-) in bladder infection with certain bacteria whereas (+) dipstick test for leukocytes doesn't always mean infection is present. Dipstick test is only useful when you have other urinary symptoms such as frequency, burning/painful voiding or change in urine characteristics (odor, color, etc.). (+) urinary symptoms & (+) for both leukocytes & nitrite then UTI is likely present.

There might be white blood cells in your urine but no bacteria