ColorMix is a tool a Supervisor I had passed me a while back. It is used to give an image the color of another while maintaining the original luminance.

You have to connect Color_In and Luminance_In for it to work. You can also choose the colorspace you want and how much you want it to mix with the original image (the one connected to Color_In)
Expand ColorMix
Group {
inputs 0
name ColorMix1
onCreate "\nif nuke.thisNode()\['f_created'].getValue()==True:\n nuke.thisNode()\['f_created'].setValue(False)\n nuke.thisNode().showControlPanel()"
tile_color 0x263bffff
label "merge color only"
selected true
xpos 208
ypos -213
addUserKnob {20 User}
addUserKnob {41 colorspace_out l colorspace T Colorspace_luma_in.colorspace_out}
addUserKnob {26 ""}
addUserKnob {41 mix T Mix.mix}
addUserKnob {26 ""}
addUserKnob {41 maskChannelMask l mask -STARTLINE T Mix.maskChannelMask}
addUserKnob {41 inject -STARTLINE T Mix.inject}
addUserKnob {41 invert_mask l invert -STARTLINE T Mix.invert_mask}
addUserKnob {41 fringe -STARTLINE T Mix.fringe}
addUserKnob {6 f_created l "First Created?" +INVISIBLE +STARTLINE}
f_created true
}
Input {
inputs 0
name Mask
xpos 320
ypos -1
number 2
}
Input {
inputs 0
name Luminance_In
xpos 494
ypos -270
}
Dot {
name Dot5
xpos 528
ypos -175
}
set N1044ee30 [stack 0]
Colorspace {
colorspace_out HSV
name Colorspace_luma_in
xpos 686
ypos -178
}
set N10454900 [stack 0]
Input {
inputs 0
name Color_In
xpos 799
ypos -273
number 1
}
Colorspace {
channels {{{Colorspace1.channels}}}
colorspace_in {{Colorspace_luma_in.colorspace_in}}
illuminant_in {{Colorspace_luma_in.illuminant_in}}
primary_in {{Colorspace_luma_in.primary_in}}
colorspace_out {{Colorspace_luma_in.colorspace_out}}
illuminant_out {{Colorspace_luma_in.illuminant_out}}
primary_out {{Colorspace_luma_in.primary_out}}
bradford_matrix {{Colorspace_luma_in.bradford_matrix}}
maskChannelMask {{{Colorspace1.maskChannelMask}}}
maskChannelInput {{{Colorspace1.maskChannelInput}}}
invert_mask {{Colorspace_luma_in.invert_mask}}
unpremult {{{Colorspace1.unpremult}}}
invert_unpremult {{Colorspace_luma_in.invert_unpremult}}
mix {{Colorspace_luma_in.mix}}
name Colorspace_color_in
xpos 799
ypos -178
bookmark {{Colorspace_luma_in.bookmark}}
}
set N104723a0 [stack 0]
Copy {
inputs 2
from0 rgba.red
to0 rgba.red
name Copy1
xpos 799
ypos -86
}
push $N10454900
push $N104723a0
Copy {
inputs 2
from0 rgba.blue
to0 rgba.blue
name Copy3
selected true
xpos 694
ypos -88
}
Switch {
inputs 2
which {{Colorspace_luma_in.colorspace_out>8?1:0}}
name Switch1
xpos 758
ypos -38
}
Colorspace {
channels {{{Colorspace1.channels}}}
colorspace_in {{Colorspace_luma_in.colorspace_out}}
illuminant_in {{Colorspace_luma_in.illuminant_out}}
primary_in {{Colorspace_luma_in.primary_out}}
colorspace_out {{Colorspace_luma_in.colorspace_in}}
illuminant_out {{Colorspace_luma_in.illuminant_in}}
primary_out {{Colorspace_luma_in.primary_in}}
name Colorspace_out
xpos 758
ypos -5
}
push $N1044ee30
Merge2 {
inputs 2+1
operation copy
output {rgba.red rgba.green rgba.blue -rgba.alpha}
maskChannelMask -rgba.alpha
name Mix
xpos 494
ypos -1
}
Output {
name Output1
xpos 494
ypos 100
}
end_group