target_color
'rgb(200,200,200)'
'rgb(190,29,209)'

target_hue   ~   target_saturation  ~ target_lightness   ~   hex_value
10          ~    40        ~         40           ~         a567ff

SET @rgb := 'rgb(190,29,209)' ;

SELECT @rgb
     , @rgbv := SUBSTRING_INDEX(SUBSTRING_INDEX(@rgb,'rgb(',-1),')',1) AS `@rgbv`
     , @v1 := SUBSTRING_INDEX(@rgbv,',',1)                             AS `@v1`
     , @v2 := SUBSTRING_INDEX(SUBSTRING_INDEX(@rgbv,',',2),',',-1)     AS `@v2`
     , @v3 := SUBSTRING_INDEX(SUBSTRING_INDEX(@rgbv,',',3),',',-1)     AS `@v3`
     , 0 + @v1  AS R
     , 0 + @v2  AS G
     , 0 + @v3  AS B

GREATEST(0+@v1,0+@v2,0+@v3) - LEAST(0+@v1,0+@v2,0+@v3)

IF(0+@v1>0+@v2
  ,IF(0+@v1>0+@v3,0+@v1,IF(0+@v2>0+@v3,0+@v2,0+@v3))
  ,IF(0+@v2>0+@v3,0+@v2,0+@v3)
  ) AS `max(R,G,B)`

IF(0+@v1<0+@v2
  ,IF(0+@v1<0+@v3,0+@v1,IF(0+@v2<0+@v3,0+@v2,0+@v3))
  ,IF(0+@v2<0+@v3,0+@v2,0+@v3)
  ) AS `min(R,G,B)`

SELECT s.R
     , s.G
     , s.B
     , IF(s.R>s.G,IF(s.R>s.B,s.R,s.B),IF(s.G>s.B,s.G,s.B)) AS `max(R,G,B)`
     , IF(s.R<s.G,IF(s.R<s.B,s.R,s.B),IF(s.G<s.B,s.G,s.B)) AS `min(R,G,B)`
  FROM (
         SELECT t.rgb
              , @rgbv := SUBSTRING_INDEX(SUBSTRING_INDEX(t.rgb,'rgb(',-1),')',1) AS `@rgbv`
              , @v1 := SUBSTRING_INDEX(@rgbv,',',1)                              AS `@v1`
              , @v2 := SUBSTRING_INDEX(SUBSTRING_INDEX(@rgbv,',',2),',',-1)      AS `@v2`
              , @v3 := SUBSTRING_INDEX(SUBSTRING_INDEX(@rgbv,',',3),',',-1)      AS `@v3`
              , 0 + @v1  AS v1
              , 0 + @v2  AS v2
              , 0 + @v3  AS v3
           FROM mytable t
       ) s

select
   substring(rgb, locate('(',rgb)+1,locate(',',rgb) -1 - locate('(',rgb)) as red,
   substring(substring_index(rgb,',',2) , locate(',',rgb)+1) as green,
   substring(rgb, locate(',',rgb, locate(',',rgb)+1 )+1 , locate(')',rgb)
                   -1 - locate(',',rgb, locate(',',rgb)+1 )) as blue

red  green  blue
190  29     209