I wasn't sure if I should make this guide by separating colors and brushes, however some islands have both colors and brushes on them. So I'll be going by islands and you can control+f to find which one you are looking for.

If you are starting fresh, this is how your tool bar should look:
without counting the tools you start with, you'll be looking for 18 items together.
10 islands in total to search.

IMPORTANT:
Please be aware that these screenshot take place early in the game. as you progress the colors of the the floors, walls, islands, and even the colors in your tool bar change.

This island is where you first started in Art Sqool.

Item 1: Color Blue
If you just started playing the game the very first object you see is color blue.
Item 2: Mop Brush
If you move left on the map you'll see this round sculpture. Next to that there is a pink wall, on top of it you'll find the "mop brush".
Item 3: Eraser
On top of the that pink wall that you're on, you should see blue stairs. The item is ontop of the stairs.
Item 4: Dot Pen
This is located behind the same stairs you found the eraser.

The 2nd Island is connected to the first island. These items should'nt that hard to find seeing as they're next to landmarks.

Item 5: Thiq Brush
This item is next to this purple frog.

Item 6: Rainbow Pencil
This item is in a door/path way next to the yellow landmark.

Item 7: Shape Maker
This item can be found behind behind that giant purple, black, and blue building.

Island 3 is right next to the 2nd island.
Item 8: Color Pink
found under purple hut.

Island can be found next to island 3.

Item 9: Paint Bucket
I have highlighted this item as it was incredible easy to see for me, but not so much fromscreenshot perspective.

Island 5 is right above island 4. The island is so small that both items can be seen.

Item 10 + 11: Square Brush and Color Purple.

Island 6 is right above island 5:

Item 12: Grease Pencil
this item is found right underneath what I can only assume as a rock climbing wall.

Item 13: Watercolor Brush
this item is found behind the purple house.
NOTE: the brush looks almost identical to the debris on the ground. just hit anyone of them and you should find it.

You should see island 7 a little in the distance higher up from island 6.

Item 14: Yellow Color
As seen in the picture. The color is that little yellow sphere.

Island 8 is directly under island 7.

Item 15: Wiggle Pencil
The wiggle pencil is found over the checkerboard blanket.
Item 16: Stickers
As you may notice in the picture above i have marked out a hole. Go next to that hole and hold the 'Page Down' key [NUM 3 if you don't have it]. There you should find the sticker brush just hiding in the hole.

Island 9 is found under island 1.

Item 17: Text Tool
This one is pretty easy. It's found right in front of the house.

Island 10 is a chess board found under the islands. You can't miss it.
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Item 18: Color White
found right on the island.

If you followed this guide correctly and congratulations, you now have all 18 tools. Your tool bar should look like this:
Happy Painting Froshmen!