Clear Coolant?

This is strange. When I change the coolant in my 1984 300 SD it looks bright green, for just a few days, and then it becomes clear... no green, a little light brown, but mostly clear. No matter how many
times I change the coolant, this same thing happens. No oil in the coolant, just clear (light brown). Any idea what causes this?
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Does it look clear in the tank or is it clear when you drain it? Richard

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