一、两个三维坐标的距离

r3 =( r1 - r2) / 256
g3 = (g1 - g2) / 256
b3 = (b1 - b2) / 256

diff = sqrt(r3 r3 + g3 g3 + b3 * b3)
值越大，相似度越小；值越小，相似度越大！也可以把上面用1减去，保持值和相似度一致

百分比就是上面得到的值除以那个值的极限最大值。sqrt(r3 r3 + g3 g3 + b3 b3)/sqrt(255255+255255+255255)

二、VB参考代码

Private Function Minimum(ParamArray Vals())
Dim n As Integer, MinVal
On Error Resume Next
    MinVal = Vals(0)
    For n = 1 To UBound(Vals)
        If Vals(n) < MinVal Then MinVal = Vals(n)
    Next n
    Minimum = MinVal
End Function
Private Function Maximum(ParamArray Vals())
Dim n As Integer, MaxVal
On Error Resume Next
    MaxVal = Vals(0)
    For n = 1 To UBound(Vals)
        If Vals(n) > MaxVal Then MaxVal = Vals(n)
    Next n
    Maximum = MaxVal
End Function

Private Sub c2hsb(ByVal clr As Long)
Dim MyR As Single, MyG As Single, MyB As Single
Dim Max As Single, Min As Single
Dim MyS As Single
Dim Delta As Single, MyVal As Single
Dim cc As String * 6
Dim r1, g1, b1 As Byte
On Error Resume Next
    cc = Right("000000" + Hex$(clr), 6)
    b1 = Val("&H" + Left(cc, 2))
    g1 = Val("&H" + Mid(cc, 3, 2))
    r1 = Val("&H" + Right(cc, 2))
    MyR = r1 / 255: MyG = g1 / 255: MyB = b1 / 255
    Max = Maximum(MyR, MyG, MyB)
    Min = Minimum(MyR, MyG, MyB)
    hsbB = Int(Max * 100)
    If Max <> 0 Then
        MyS = (Max - Min) / Max * 100
    Else
        MyS = 0
    End If
    hsbS = MyS
    If hsbS = 0 Then
        hsbH = 0
    Else
        Delta = Max - Min
        Select Case Max
        Case MyR
            MyVal = (MyG - MyB) / Delta
        Case MyG
            MyVal = 2 + (MyB - MyR) / Delta
        Case MyB
            MyVal = 4 + (MyR - MyG) / Delta
        End Select
        MyVal = MyVal * 60
        If MyVal < 0 Then MyVal = MyVal + 360
        hsbH = MyVal
    End If
'   Debug.Print "hsb="; hsbH; " "; hsbS; " "; hsbB
End Sub

Private Function ColorDistance(ByVal c1 As Long, ByVal c2 As Long) As Long
Dim cd As Long
Dim h2, s1, b1, h3, s2, b2 As Single
On Error Resume Next
    If c1 = -1 Or c2 = -1 Then
        ColorDistance = 1000000
        Exit Function
    End If
    c2hsb (c1)
    h2 = hsbH / 360
    s1 = hsbS
    b1 = hsbB
    c2hsb (c2)
    h3 = hsbH / 360
    s2 = hsbS
    b2 = hsbB
    cd = Abs(h2 - h3)
    cd = cd + Abs(s1 - s2)
    cd = cd + Abs(b1 - b2)
    ColorDistance = cd
End Function