If you enabled FilterLite but the game looks the same, or a preset looks strange, work through these checks.
Filter Not Showing at All
1. Check where you enabled it. In a shader loader such as Iris or OptiFine, look under Options → Video Settings → Shader Packs. If your copy is in the resourcepacks folder, it must be in the Selected column under Options → Resource Packs.
2. Make sure shaders are switched on. Some loaders have a master shader toggle. If shaders are set to off, no pack will display.
3. Check that another shader pack isn't selected instead. Only one shader pack can be active at a time in a loader.
4. Reload. Leave the menu fully with Done so the game reloads, then look again.
Preset Looks Wrong or Too Strong
1. Confirm which preset is active. Open the pack's settings. It's easy to leave a strong style selected from earlier testing.
2. Try a different preset. There are 20 to choose from. If one is too heavy for regular play, switch to a gentler style.
3. Check your other resource packs. Packs that change textures or colors stack with the filter and can make results look odd. Temporarily disable them to compare.
4. Try another graphics mode. FilterLite is meant to work with standard Minecraft graphics. If something looks off, switch Graphics in Video Settings and see if it improves.
If all else fails, disable and re-enable the pack to reset it.