I'm using CS2 so I'm not sure if this works in PS7, but this is the way that I would change the black lines in a black and white image to another colour.

First make sure that the image is in RGB or CYMK (Image>Mode>RGB Colour or CYMK Colour)
Then add a Channel Mixer adjustment layer (Layer>New Adjustment Layer>Channel Mixer)
Then just start adjusting the sliders till you find the colour that you want.
Adjusting the "Constant" slider helps keep the white of your image white.
For some additional adjustments you could add a Hue and Saturation Adjustment Layer on top of your Channel Mixer.

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