list = `convert 9784152097712_10.jpg -format %c histogram:info:`.split("\n")
rgbs = list.map { |line| $1 if line =~ /\s+#([\dA-F]+)\s+/ }
num_non_gray = rgbs.select { |rgb|
  r = rgb[0,2]; g = rgb[2,2]; b = rgb[4,2]
  r != g || g != b
}.count

num_non_gray > 0 # true: 正しい画像, false: (一色かどうかはわからないが)グレー画像

# output example: convert 9784152097712_10.jpg -format %c histogram:info:
==begin
         1: (172,172,172) #ACACAC srgb(172,172,172)
         1: (174,174,174) #AEAEAE srgb(174,174,174)
         1: (175,175,175) #AFAFAF srgb(175,175,175)
         4: (177,177,177) #B1B1B1 srgb(177,177,177)
         7: (178,178,178) #B2B2B2 srgb(178,178,178)
        11: (179,179,179) #B3B3B3 grey70
        29: (180,180,180) #B4B4B4 srgb(180,180,180)
        22: (181,181,181) #B5B5B5 grey71
        25: (182,182,182) #B6B6B6 srgb(182,182,182)
        13: (183,183,183) #B7B7B7 srgb(183,183,183)
         8: (184,184,184) #B8B8B8 grey72
         3: (185,185,185) #B9B9B9 srgb(185,185,185)
         1: (186,186,186) #BABABA grey73
         4: (187,187,187) #BBBBBB srgb(187,187,187)
         1: (188,188,188) #BCBCBC srgb(188,188,188)
         1: (189,189,189) #BDBDBD grey74
         3: (191,191,191) #BFBFBF grey75
         2: (194,194,194) #C2C2C2 grey76
         2: (196,196,196) #C4C4C4 grey77
         1: (197,197,197) #C5C5C5 srgb(197,197,197)
         1: (198,198,198) #C6C6C6 srgb(198,198,198)
         2: (200,200,200) #C8C8C8 srgb(200,200,200)
         2: (201,201,201) #C9C9C9 grey79
         2: (204,204,204) #CCCCCC grey80
         1: (207,207,207) #CFCFCF grey81
         1: (210,210,210) #D2D2D2 srgb(210,210,210)
         1: (211,211,211) #D3D3D3 LightGray
==end