Converting a color photo into grayscale

[Larry]

Okay I have a color elevation map that I would like to convert into grayscale. But when I do change it to grayscale I need a certain color on the elevation to be white but it ends up a very dark color. I'm wondering if there is a tool in/for photoshop that lets me select a particular RBG color and change it to another color across the whole picture.

In simple words I want to make all R:198 G:194 B:150 into pure white on the whole map without going around with a brush painting it manually.

[Easyscor]

There should be, at least, there's one in Gimp and it's a copy cat.

I just remembered, in Gimp, it's called the Select by color tool. Doh.

[Larry]

Yeah I looked around and found it. In PhotoShop its call, get this, "Replace Color". 

[Dux]

It may also be possible (but not always) to just replace the palette while the file is in Indexed Color mode.

Or... depending on what colors the original gradient is made up of, you can sometimes extract a decent greyscale by looking at each of the R, G, or B channels in isolation. Open your Channels palette for that.