defining custom colors in surface plot legend

I have a surface plot (2-D contour type) with 8 entries in the legend. The default colors are in random order, but I want the range to be displayed in a logical ranking from hot to cold. So I plan to use a simple grayscale from white to black with shades of gray in between.

In VBA I enter RGB values like this:

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The problem is that Excel rounds these numbers to the nearset color in the 56 color palette. In other words all RGB from 140 to 150 are the same shade of gray, there is no graduation.

How do I disable the rounding? Do I need to create custom colors in Excel? If so how do I do that? Do new colors replace the default 56? Are they present only in the current workbook?

thanks shg

the ActiveWorkbook.Colors command was what I was missing

the only problem is if another chart in your spreadsheet uses a color that you delete by defining new greys..guess there is no way around that and why Excel 2007 allows 4 billion color definitions!