It can be useful when designing interfaces to use one color hue and then darker and lighter shades of that color. Using the HSL color scheme this is possible and quite easy. For more information on the HSL color scheme see the RGB to HSL colors, and Back! Tech Tip.

The following code takes in as parameters the pointers to RGB color values of a base color and then a real shade value from -1 to 1 to decide whether to darken or lighten the value, and how much to do so. -1 will darken the color to black and 1 will lighten the color to white. The RGB_to_HSL and HSL_to_RGB methods are from the Tech Tip referenced above, RGB to HSL colors, and Back!

RGB shade darkening method
Parameters:
$1, $2, $3: Pointers to R, G, B integer values 0 to 255, afterwards these values contain the new shaded RGB values.
$4: Real value -1 to 1 defining the desired shading. Negative numbers shade darker and positive numbers shade lighter.

C_POINTER($1;$2;$3) C_LONGINT($color_R_l;$color_G_l;$color_B_l;$color_H_l) C_REAL($4;$shade_r;$color_S_r;$color_L_r) $color_R_l:=$1-> $color_G_l:=$2-> $color_B_l:=$3-> $shade_r:=$4 RGB_to_HSL($color_R_l;$color_G_l;$color_B_l;->$color_H_l;->$color_S_r;->$color_L_r) $color_L_r:=Choose($shade_r>0;$color_L_r/(1+$shade_r);     Choose($color_L_r*(1-$shade_r)>1;1;$color_L_r*(1-$shade_r))) $color_S_r:=Choose($shade_r<0;$color_S_r/(1-$shade_r);     Choose($color_S_r*(1-$shade_r)<1;1;$color_S_r*(1-$shade_r))) HSL_to_RGB($color_H_l;$color_S_r;$color_L_r;$1;$2;$3)

Note: Two lines in the above code include line breaks. Be careful when copying and pasting code.