I'm trying to think of a way of determining whether the mouse is currently over an isometric tile without selecting the adjacent tiles. With Allegro 4 I could create a color mask in a sub bitmap and use get_pixel to find where the mouse was. Is there a way to do this with Allegro 5?

Allegro 4/5/DirectX/et al doesn't really change how you do it.

https://stackoverflow.com/questions/6915555/how-to-transform-mouse-location-in-isometric-tiling-map

https://gamedev.stackexchange.com/questions/38320/is-it-possible-to-map-mouse-coordinates-to-isometric-tiles-with-this-coordinate

http://archive.gamedev.net/archive/reference/programming/features/mm4ihm/index.html

al_get_pixel(ALLEGRO_BITMAP *bitmap, int x, int y) ??

Yes, possibly :/ Can Allegro 5 do something like this?

I always wanted to make an isometric game, problem is I could not draw a sprite if my life depended on it . and you never find enough graphics for an isometric game.
Top down graphics are much easier to get.

Here's some tiles I made myself, you're welcome to use them if you wish, at least to get you started. The roads are in the same style as SimCity2000, but they're not copied from that game, I made them myself from scratch (no copyright issues).

There's tutorials online, I suggest looking up tutorials on making tiles with Blender. I am not much of an artist either, but creating 3D renders is easier and perfect for isometric tiles. Another good program that helps create them easier is Inkscape, there are tutorials for it as well.

{"name":"611311","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/3\/a\/3a6c816125706c8159fd3cb7fcbb72df.png","w":640,"h":256,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/3\/a\/3a6c816125706c8159fd3cb7fcbb72df"}

I organized these tiles in a binary fashion. So the north corner is 1, the east corner is 2, the south corner is 4 and the west corner is 8. To get the tile number you want, just figure out which corners you wish to be raised, so if you want the north and east corner, that's tile #3 (1 + 2). The roads are organized in a similar fashion. I done it this way so that in my Isomed editor you can just draw the roads and tiles and it will use binary logic to select and combine tiles (I done something like this in my Deluxe Pacman 2 editor to draw as well).

This is an example of how these look. Believe it or not, I created these tiles just with Microsoft's Paint, that's it.

{"name":"611312","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/3\/0\/30d213ee3764963ecda394a09d720c36.jpg","w":495,"h":269,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/3\/0\/30d213ee3764963ecda394a09d720c36"}

You're talking about a mousemap (search online for that term).

I used to use one with my program, this one to be exact (with the tiles I posted above if you want it)...

{"name":"611314","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/a\/3\/a3043314ff5c121b96d07e5ad63cae05.png","w":640,"h":128,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/a\/3\/a3043314ff5c121b96d07e5ad63cae05"}

It's a fairly simple technique, just figure out which tile you are clicking on, which tile number is at that location, then check the identical mousemap. You just check the colour at the location your mouse clicked on and go from there. If the colour is red, you clicked on a tile north of the position your mouse is on (actually, north-west, depending on how you draw them). If you clicked on cyan, you go one tile south, blue is north-west etc.

I have not bothered to try coding this with Allegro 5, though it should be simple. I used Allegro 4 for my Isometric project (it's an older one).

The way I did it was I loaded two totally separate bitmaps, the main graphics and the mousemap (I provided some above). Once you know what tile you are in (just a general idea, the square area, not the isometric one), you then check which tile type if there then check the equivalent mousmap tile for the colour.

You just need an Allegro function where you can check the colour of a pixel on a bitmap. In this case, I have a 32bit PNG file for the mousemap.

I have not done this for Allegro 5 yet, but I would think you need to lock the bitmap for reading only, then call a function to read a pixel and check the colour. Should be pretty straight forward to be honest.