And while I have done that on several occasions (and will continue to do so, as and when time allows), I thought that it might be a good idea to show you how you can do something like that yourself (quickly and easily) - just using free software (for personal use only of course).
So, here goes :) :) :)
OK then, the very first thing you'll need to do is download and install GIMP (it's a free alternative to something like photoshop).
Once it's installed, open up GIMP and choose the FILE > OPEN option to locate the relevant pdf file on your computer (for the purpose of this first example I'll be using my recent sewer tiles).
You should then get a dialogue box appear that looks something like this:
From here you will first need to select the page that you want to edit (in this example I've chosen page 5) - plus you will also need to input a resolution of 300 pixles/inch, and make sure that the anti-aliasing box is NOT checked.
And that will ensure that the image is imported correctly ...i.e. like this:
From here all you'll need to do is select the COLORS > COLOR BALANCE option (from the menu bar), and move the three sliders around (in the ADJUST COLOR LEVELS section of the dialogue box) until you get the desired colour.
And, as you can see in the above example, I've moved the top slider over to the right (towards RED), and the bottom two sliders over to the left (towards MAGENTA and YELLOW respectively) to change the water texture to something that you could probably use as a pool of blood.
However, if you don't want to change the colour of the whole image you can also use the 'Rectangle select tool' (outlined in red in the top left corner), and use that to draw/select a box around the object you would like to alter...
...like I've done with one of the bottle textures in the above example.
Then, when we alter the colour balance, the only thing that will change is the selected area ...like you can see below.
So yeah, if you ever find that you're in need of a slightly different colour dungeon tile, or bottle, or anything like that, then this is one way of doing that without much fuss (though if you're feeling particularly adventurous, you can also play around with some of the other options in the COLOR menu for even more variation ;) ).
Anyway, I hope this has been of help - but if you have any issues, please feel free to get in touch.
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IMPORTANT NOTE: As I said above, please feel free to alter the textures as much as you like for your own personal use ...however please do not share any variations of these files publicly. Thank you.