The standard blending equation is:

```
out = alpha * new + (1 - alpha) * old
```

Where `out`

, `new`

and `old`

are RGB colors, and `alpha`

is a floating point number in the range [0,1].

So, you have (for red):

```
240 = 0.1 * newR + 0.9 * 255
```

Solving for `newR`

, we get:

```
newR = (240 - 0.9 * 255) / 0.1
```

which evaluates to 105. Repeat for the other components, and you’re done.