One UI - All big companies removed there colors in there apps

Why google, facebook, truecaller
removed the colors of there apps to (white + grey) or black

Microsoft just added color back to its Windows 10 icons, actually.

Welcome to the forum! Interesting question.

Google and Apple both embraced that flat look in recent years and encouraged app makers to adapt their design guidelines for a uniform look of their platforms. They say similar things about limited/conscious use of color in their guides:

From Google’s Material Design Guide:

Limiting color
By limiting the use of color in your app, the areas that do receive color gain more attention, such as text, images, and individual elements like buttons.

A grayscale color palette is best for allowing photography and text to stand out.

From Apple’s Human Interface Guidelines

In general, choose a limited color palette that coordinates with your app logo. Subtle use of color is a great way to communicate your brand.

Use color judiciously for communication. The power of color to call attention to important information is heightened when used sparingly. For example, a red triangle that warns people of a critical problem becomes less effective when red is used elsewhere in an app for noncritical reasons.