What you want to do is make sure nothing in your texture stays in gray scale after you tint it, meaning before you put it in the plt editor. To do that, you need to shy away from white (256 256 256 [Edit: 255 in most scales]) and black (0, 0, 0).
To make sure you keep away from those, in your texture editor of choice, simply do a color replace. Many programs have you select the replacee as the foreground, and the replacer as the background, then you just paint with the replace tool and it exchanges one to the other. Some programs also let you do this with a percent of lenience.
So what I do is select 0,0,0 and change my replacement color to 4,4,4 or 8,8,8 gray. Likewise with white, I select 256 256 256 white, and replace it with something slightly dimmer, say 240 or 250.
Using the replacer tool, I just spraypaint it on with the tool set to the largest size and the heaviest fullest grain (solid). This generally does half to all of my image at once.
Save your image, and import into your favorite PLT editor. You should be able to avoid that grayscale glitch after that.
I don't know for sure if GIMP has that issue or not, but the same logic would be used to counter it.
You might also do a contrast shift of the entire image to squeeze the color into acceptable boundaries. I think a contrast/brightness shift of 2% would be sufficient. [Edit: I mean contrast down, which takes in the range max and min, or a combination of contrast down with a brightness up if you want your dark shifted more than your brights. There are many other histogram type movements you can do to achieve the same outcome, based on program.]
Another thing you want to do is make sure that any image you import into the older PLT editors is actually gray scale. Any one pixel with color outside the gray scale range will make a mess.
