The reason to write this post was a simple desire to code something and write about it :)

So, the task is to implement transition color effect. We have to make an object smoothly change its color from some initial color to some target color during fixed time. We will use RGB color space.

In RGB space a color is a 3D point. The first coordinate is red component of the color, the second one is green and the third one is blue. On the input of our transition color algorithm we have the initial color $S(R_S, G_S, B_S)$, the target color $T(R_T, G_T, B_T)$ and duration of the transition $D$.

We are going to reach the target color from the initial color in $N$ steps. Let's name them as transition steps.

Let $d_i$ be the duration of $i$-th transition step. Here are some properties of $d_i$: $$\begin{array}{c} d_i \leq D; \\ \sum\limits_{i = 1}^N d_i = D; \end{array}$$

Thus, $\frac{d_i}{D}$ is a part of the whole way from the initial color to the target color contributed by $i$-th transition step. After $i$ transition steps we have walked $F_i$ of the whole path where

$$F_i = \sum\limits_{j = 1}^i \frac{d_i}{D};$$

Let $W_i$ be the color on the $i$-th transition step, then

$$\begin{array}{c} W_i = (R_i, G_i, B_i); \\ R_i = R_S + F_i \cdot (R_T - R_S); \\ G_i = G_S + F_i \cdot (G_T - G_S); \\ B_i = B_S + F_i \cdot (B_T - B_S); \end{array}$$

To make the transition color effect we are just walking through the $N$ transition steps until $W_i = T$. This can be easily implemented as a computer algorithm. Here is my implementation. Let's take a look at the screenshot of the UI:

The big rectangle shows the current color, the small one shows the target color. We can change the target color with the sliders. After pressing the "Go" button, the big rectangle will smoothly change its color according to the given target color.

That's it!