When looking for a slight change in lightness I usually do the math - simple counting really.
The hex format for colours is
RRGGBB, meaning red, green and blue. Hexadecimal is counted from 0-F (so after 9 comes A).
If I have
#191970 as my main button colour I'll add 1 or 2 to each colour value, resulting in a similar but lighter colour. Adding 1 to each would result in
The same could be done for finding a darker shade. Subtracting 1 from each colour value in
#191970 would result in
However, adding or subtracting 1 or 2 from each colour value is likely to result in an indistinguishable difference, so you'll probably need to add or subtract atleast 10 from each.
If you're using the number format
(255, 255, 255), you can just add to each number using normal base 10 operations.
Adding the same amount to each colour value ensures that the hue and saturation remain consistent and the resulting colours blend harmoniously in a design.