adding color to grey scale video

If its grey scale, you cannot colourise it without the colour information, unless you are trying to colourise it for some other meaning?

The video data represents temperature for each pixel so I want to, say for instance, if a particular pixel is above a certain temperature, make that pixel red, or a group of pixels to represent a temp gradient, how would I do that.

what format is your video output? is it RGB or YCbCr?

YCbCr is the current format.

You will need to take your RGB value and convert it to YCbCr via a colour space converter, or just have a set of preset YCbCr values. Then you will need to convert it to 444 to 422 video for output (assuming you're using NTSC or PAL).

(Y ) = (  0.299 ,  0.587 ,  0.114 , 0.0   ) (R)
(Cb)   ( -0.147 , -0.289 ,  0.436 , 128.0 ).(G)
(Cr)   (  0.615 , -0.515 , -0.1   , 128.0)) (B)
                                            (1)

Ok, I must be missing something. I have YCbCr data in RAM that is 8bit grey scale. If I have YCbCr grey scale how would I colorize it I guess is the question. My output goes to an encoder that outputs NTSC analog.

For NTSC, YCbCr is not trivial to colour as it is. It will usually be running with Y interleaved with alternating Cb and Cr. This is because the human eye only needs half of the colour information compared to luminance, so you get the following data stream: